Facebook is a social networking service and website where sers must register and thereafter may create a personal profile, add other users as friending friends, and exchange messages, including automatic notifications when they update their profile.
Additionally, 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".
The name of the service stems from the Facebook (directory) which is the colloquial name for the book given to students at the start of the academic year by some university administrations in the United States to help students get to know each other.
Facebook allows any users who declare themselves to be at least 13 years old to become registered users of the site.
Launched in February 2004 Facebook is operated and privately owned by Facebook Inc. and was founded by Mark Zuckerberg with his college roommates and fellow students Eduardo Saverin, Dustin Moskovitz and Chris Hughes. As of February 2012, Facebook has more than 845 million active users.