ENLARGE On Twitter, Parisians used the hashtag “#PorteOuverte”—or “open door” in French—to offer shelter to stranded visitors. Photo: Eric Thayer/Reuters 
 
 Social-media companies Facebook Inc. FB 1.05 % and Twitter Inc. TWTR -0.71 % provided new tools allowing people around the world to track and discuss the violent attacks in Paris on Friday with unprecedented speed and depth. 
 
 Facebook activated its “Safety Check” feature Friday, allowing the site’s users in the area of the attacks to mark themselves as safe on their profiles. This marked the first time the tool was enabled for violent attacks. Twitter’s news-curation tab, known as “Moments,” featured tweets, images and videos from news agencies and bystanders that showed snippets of the attacks’ aftermath. 
 
 Alphabet Inc.’s Google said it is making international calls to France from its Hangouts mobile communication app free through the weekend, so people can check on the status of friends and relatives. The free calls can be made through Google’s Hangouts app on Android phones and iPhones. 
 
 On Twitter, Parisians used the hashtag “#PorteOuverte”—or “open door” in French—to offer shelter to stranded visitors. On Periscope, the live-streaming app owned by Twitter, a user posted a video of the scene around the Bataclan concert hall, the site of one of the attacks. Other users posted live videos of their reaction to the violence, often with police sirens blaring in the background. 
 
 “My thoughts are with everyone in Paris tonight,” Facebook CEO Mark Zuckerberg said on his Facebook page. “Violence like this has no place in any city or country in the world. We’ve activated Safety Check, so if you’re in Paris you can mark yourself safe or check on your friends and family.” 
 
 Facebook’s safety check tool been activated five times since its introduction in October 2014. The feature is activated by the company’s “social good” team, which is tasked with building features for Facebook that help people in crisis. The social network was spurred to develop the product after the 2011 earthquake and tsunami in Japan, Mr. Zuckerberg has said. 
 
 Facebook sends users who are detected in a crisis zone a message asking if they are safe. The users can then use the tool to indicate on their profiles that they are safe. Users can also mark others as safe or keep tabs on other Facebook connections in the affected area. The information is only visible within a user’s Facebook network. 
 
 Facebook first activated the tool on April 25 after a big earthquake hit west of Nepal’s capital, Kathmandu. More than seven million people in the area were marked safe, notifying more than 150 million people connected to them, Mr. Zuckerberg said in a later post. 
 
 Facebook turned it on again after a powerful quake struck Chile in September. The feature was also enabled twice in October, after Hurricane Patricia hit Mexico and when an earthquake shook Afghanistan, Pakistan and the surrounding region. 
 
 This year, Facebook and Twitter have invested in delivering real-time news to their users. 
 
 In June, Facebook’s photo-sharing app Instagram overhauled its “Explore” tab, making it easier for users to find images by place and hashtag. “People are hungry for what’s happening right now in the world,” Instagram CEO and co-founder Kevin Systrom told The Wall Street Journal at the time. 
 
 In October, Twitter unveiled “Moments,” a slimmed-down version of its own feed with tweets selected by a team of editors. That same month, Facebook said it has indexed more than two trillion posts across the social network so users can track news events as they unfold. 
 
 Write to Deepa Seetharaman at Deepa.Seetharaman@wsj.com ||||| Facebook launched a feature so that people in Paris can let their network know they are alive on Friday after at least 128 were feared killed following multiple terror attacks across the French capital and a hostage crisis at a rock concert in the Bataclan theater that prompted President François Hollande to shut down the country’s borders and declare a state of emergency. 
 
 The social network’s “Safety Check” tool here asks users if they are in the affected area and enables them to “mark” themselves as “safe.” Then their Facebook friends will receive a notification. A “mark safe” button lets users mark others as safe, too. There is a comments section in each update, mostly filled with exclamations of relief. 
 
 This “Safety Check” was put in place earlier this year after the deadly earthquake in Nepal in April. 
 
 After Parisian police ordered residents to shelter in place and invite others inside their homes, the Twitter hashtag #PorteOuverte started trending so people could connect and find refuge. 
 
 Meanwhile, the U.S. Department of State Bureau of Consular Affairs says U.S. citizens in Paris should check-in with their families instead of with the Embassy, so that vital emergency numbers aren’t tied up. ||||| In the midst of a series of alleged terrorist attacks in Paris on Friday night that have left at least 129 dead and more than 350 wounded, some Twitter users in the city immediately began using the hashtag #PorteOuverte, or "open door," to offer their homes as shelter for those afraid to be on the streets with nowhere to go. 
 
 Social media users were tweeting invitations to their homes for those who might "need help" or a "place to sleep." Those offering said they would provide their address to those interested through direct messages. Others included their addresses in the tweets. 
 
 Read the tweets here or read some examples below: 
 
 guys if you're in paris and you don't speak french : the hashtag #porteouverte is ppl who can welcome you just dm them 
 
 Anybody who's stranded in Paris and needs shelter and somewhere safe, any Sikh Gurdwara (temple) will be happy to accommodate #PorteOuverte 
 
 Hey tourists in #Paris if you need help or some place to sleep around Jussieu 5th arrondissement, my door is open. #PorteOuverte #OpenDoor 
 
 You're welcome to our home if you're afraid of going home through Paris and are near La Vilette #porteouverte 
 
 PorteOuverte bd Davout me contacter en mp 
 
 Help #PorteOuverte 
 
 PorteOuverte boulevard de Charonne, 20eme limite 11eme. Be safe. 
 
 PorteOuverte 3 rue St Joseph 
 
 Anyone, come to Rue Keller. I can get the door open if you need. Lot of cops here. #PorteOuverte 
 
 A series of attacks Friday night took place at multiple restaurants and bars in the city. A hostage situation taking place at the Bataclan theater, where the American rock band Eagles of Death Metal were performing, led to more than 100 people being held hostage and 89 killed. An explosion occurred at the Stade de France during the France vs. Germany soccer game. 
 
 The death toll is currently at least 129 people, according to Le Monde. 
 
 Nov. 14, 2015, 1:42 p.m. Eastern: This story has been updated. ||||| By Naomi Gleit, VP of Product Management, Sharon Zeng, Product Manager & Peter Cottle, Software Engineer 
 
 In times of disaster or crisis, people turn to Facebook to check on loved ones and get updates. It is in these moments that communication is most critical both for people in the affected areas and for their friends and families anxious for news. 
 
 We want to provide a helpful tool that people can use when major disasters strike, so we’ve created Safety Check – a simple and easy way to say you’re safe and check on others. 
 
 During a major disaster, Safety Check will help you: 
 
 Let friends and family know you’re safe 
 
 Check on others in the affected area 
 
 Mark your friends as safe 
 
 Only your friends will see your safety status and the comments you share. 
 
 The 2011 earthquake and tsunami in Japan was devastating. According to the Japanese Red Cross, more than 12.5 million people were affected nationwide, and more than 400,000 people were evacuated. During that crisis we saw how people used technology and social media to stay connected with those they cared about. 
 
 Our engineers in Japan took the first step toward creating a product to improve the experience of reconnecting after a disaster. They built the Disaster Message Board to make it easier to communicate with others. They launched a test of the tool a year later and the response was overwhelming. 
 
 Unfortunately, these kinds of disasters happen all too frequently. Each time, we see people, relief organizations and first responders turn to Facebook in the aftermath of a major natural disaster. 
 
 These events have taught us a lot about how people use Facebook during disasters and we were personally inspired to continue work on the Disaster Message Board to incorporate what we’ve learned. This project soon became Safety Check, which will be available globally on Android, iOS, feature phones and desktop. 
 
 The team set out to create a simple and easy-to-use tool that allows people to connect with their network of friends and family when it matters most. 
 
 Here’s how it works: 
 
 When the tool is activated after a natural disaster and if you’re in the affected area, you’ll receive a Facebook notification asking if you’re safe. 
 
 We’ll determine your location by looking at the city you have listed in your profile, your last location if you’ve opted in to the Nearby Friends product, and the city where you are using the internet. 
 
 If we get your location wrong, you can mark that you’re outside the affected area. 
 
 If you’re safe, you can select “I’m Safe” and a notification and News Feed story will be generated with your update. Your friends can also mark you as safe. 
 
 If you have friends in the area of a natural disaster and the tool has been activated, you will receive a notification about those friends that have marked themselves as safe. Clicking on this notification will take you to the Safety Check bookmark that will show you a list of their updates. 
 
 If you’re ever in a situation that would require you to use Safety Check, we hope it’s a tool that helps you stay connected to those you care about, and gives you the comfort of knowing your loved ones are safe. 
 
 Update as of 11/19/15 
 
 We created Safety Check to help in times of crisis and activated it eight times between October 2014 and November 2015 after natural disasters including the recent earthquakes in Nepal. The Paris attacks in November 2015 was the first time we activated the tool for a crisis other than a natural disaster. Read more from our Vice President of Growth Alex Schultz about why we made that decision and how it impacts our approach to Safety Check going forward. 
 
 Update as of 11/17/2016 
 
 Initially, Facebook decided when Safety Check was activated, but our goal has always been to make it more useful for people for more types of incidents. We believe people closest to a disaster should play a bigger role in deciding when Safety Check is most helpful. As of November 2016, Safety Check will be turned on by our community instead of Facebook. How this works is that once an incident occurs, a third party source alerts Facebook. As part of that process the incident is automatically titled in line with the third party alert. If a lot of people in the area are talking about the incident, they may be invited to mark themselves safe, and invite others to do the same, through Safety Check. ||||| This new craze could actually be a gift to hackers, experts warn 
 
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 A NEW craze sweeping Facebook could actually pose a huge security risk, experts have warned. 
 
 People have been sharing lists of the top 10 best concerts they’ve ever been to, including one which is a lie, encouraging pals to guess. 
 
 Getty Images 1 The fun craze has been sweeping Facebook – but could put you at risk of being hacked 
 
 But now online security experts have warned you could be secretly giving hackers clues to your passwords and answers to security questions. 
 
 Professor Alan Woodward, from the University of Surrey, urged people to think carefully about what information they were putting into the public domain. 
 
 He told the BBC: “I wouldn’t do it. 
 
 “But it’s difficult to tell people not to take part, as it is part of their social interaction and has become the norm.” 
 
 Digital analyst Brian Solis suggested the game could even be a form of “quasi-phishing” and said hackers often use “social engineering” to get sensitive information out of victims without them realising. 
 
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 He wrote on LinkedIn: “In some cases, memes can be deceptively dangerous. 
 
 “Something as simple as, ‘what was your first concert you went to?’ can lead to compromise though human hacking. 
 
 “Social engineering is the art of manipulating people out of confidential information aka secrets. 
 
 “Hackers use this technique to convert secrets into access. 
 
 Joey Graceffa thinks he may have been 'scammed' by expensive juicer 
 
 "In this case, asking about bands could be a quasi phishing scam." 
 
 It comes as experts warned against a Gmail phishing scam so “convincing” it has duped even IT experts into handing hackers their passwords. 
 
 And even Google and Facebook have admitted they were conned out of an alleged £77million in a phishing scam. 
 
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– Amid the horror of the Paris terror attacks, there was a response from ordinary citizens that the city can be proud of. With the hashtag #PorteOuverte, meaning "open door," Parisians offered shelter to tourists and others stranded by the chaos, Mic reports. People were desperate to find safe havens because they had been evacuated from buildings but had been told the streets and public transport were unsafe, French-Algerian journalist Nabila Ramdani tells the BBC. Taxi drivers turned off their meters and started taking people home for free, while there were "many others stuck in shuttered bars, restaurants, some even locked in kitchens," tweeted France24 correspondent Sophie Pilgrim. During the attacks, Facebook activated its new "Safety Check" feature for the fifth time ever and the first during a violent attack, reports the Wall Street Journal. The feature sends Facebook users in areas affected by a crisis a message asking whether they are safe, and then gives them the option of checking "Yes, let my friends know," to let their friends know they are OK. The feature was first activated after the Nepal earthquake earlier this year and was turned on during three subsequent natural disasters, the Journal notes. The Paris response could point the way for social media use during future crises, though Wired reports that there was one problem with the #PorteOuverte hashtag: So many people tweeted about how inspiring it was that users worried real offers of help might be crowded out.