Facebook is a social networking service and website launched in February 2004. As of May 2012, Facebook has over 900 million active users, more than half of them using mobile devices. Users may join common-interest user groups, organized by workplace, school or college, or other characteristics, and categorize their friends into lists such as “People From Work” or “Close Friends”. Facebook allows any users who declare themselves to be at least 13 years old to become registered users of the site. Facebook was founded by Mark Zuckerberg.