Chinese relish crack in Great Firewall, log on to Facebook SHANGHAI (Reuters) - Some Chinese Internet users have this week been able to access blocked websites such as YouTube, Facebook and Twitter, relishing the newfound freedom although the reason for the breach in China's Great Firewall of censorship was a mystery. China blocks most foreign social networking sites (SNS) out of fear that unfettered access would lead to instability. Chinese SNS firms have filled the void by offering similar products that censor topics the government may find sensitive. I can suddenly access YouTube! No need to breach the firewall! Weibo user Arvin Xie posted on Tuesday. Weibo is a microblogging platform, similar to Twitter, that allows users to post short messages and follow other users. Internet users including students on university campuses reported that they were able to access YouTube, Facebook and Twitter on their mobile phones and desktops in the afternoon and evening on Monday and Tuesday. "I used Facebook for the first time yesterday," Zhang Wenjin, 23, a student at Shanghai's prestigious Jiao Tong University told Reuters on Tuesday. I went on and took a look. I'm sure there were suddenly a lot of people who signed up on Facebook yesterday," Zhang said, adding that she had also signed up for an account. It is unclear what caused the crack in China's Great Firewall, as the blocking of websites and censoring of search results for politically sensitive terms is known, or how widespread it was. On Wednesday, access to Facebook, YouTube and Twitter was again blocked. Some users in China pay for a virtual private network (VPN) to bypass the blocking of websites and censoring of searches. Over the weekend, Chinese users also gained access to Google Inc's social networking site, Google+ and flooded U.S. President Barack Obama's page on the site with calls for greater freedom in the world's most populous country. Google+ is currently blocked through regular desktop access but its mobile application, which let users in China access the site, has become accessible recently. China, with more than 500 million Internet users, is the world's largest and most vibrant Internet community. Additional reporting by Jane Lee; Editing by Jacqueline Wong and Robert Birsel