Where The Features Came From We know what you are thinking: Wait, didn't Twitter just celebrate its sixth birthday? How is it now celebrating its anniversary? Well, on March 21, 2006, Twitter co-founder Jack Dorsey sent out the first tweet from the service, then called twttr. But on July 15, 2006, six years ago today, the service was launched publically. Twitter's core feature has remained the same over the last six years: Send messages out -- messages no longer than 140 characters -- to the world, your followers. However, since its launch in July 2006, the service has evolved, with lots of key features. Sure, the biggest change might just be the sheer volume of people that keep joining -- there are 140 million global active users and 400 million tweets are sent a day -- but here's a look back at the evolution of some of the service's now-crucial features. Mentions/@replies -- Funny enough, when Twitter launched there wasn't any real way to direct a message at a specific user or have a real conversation with one person. There is some debate over who first started the @ reply or mention model, where people place an "@" in front of a username and then place the message afterwards. However, Robert Andersen seems to be credited with being the father of the @ reply. He began using it on Nov. 2, 2006, and it started to catch on. See the full history of the @ reply here. #Hashtags -- Hashtags weren't a part of the original Twitter either. The idea was actually invented by Chris Messina, now head of the Citizen Agency. His idea was to create a place to group different tweets and he proposed the idea of using the "#" symbol. Twitter heard the call and the tags; we got the hashtag system a few months later. After that Twitter implemented the Trending Topics column on the site. #followfriday -- Speaking of hashtags, it's hard to ignore an important one like #followfriday. Every Friday people recommend others to follow on Twitter and use the #followfriday tag. Similar to the @replies and the hashtag itself, the idea was started by a Twitter user and not by Twitter itself. Micah Baldwin wanted an easier way to find people on Twitter and started up the weekly Twitter tradition. Retweet -- Retweeting is a crucial part of Twitter these days. But it wasn't always. It wasn't until March 2008 that the way of re-sharing hit the service. Users began placing a RT (Re Tweet) in front of another user's message, giving credit to the other user for the tweet. There is again debate about who started the "retweet" action first. But by August 2009, Twitter began integrating a "retweet" button into the site -- that button, as you see today, would automatically reshare the tweet with your followers. Mobile Apps -- Twitter was all about mobile phones from the start. It allowed you to send tweets via text message, thus the 140-character limit. However, it wasn't until 2008 that Twitter mobile apps started to come on line. Apps like Twitdroid for Android and Twittelator for iPhone started to pop up. Then in April 2010 Twitter acquired Atebits makers of Tweetie and rolled out its own iPhone app. It also made its own Android app. Now 60 percent of active Twitter users are active mobile users.