Facebook is a social networking service and website launched in February 2004, owned and operated by Facebook, Inc. As of May 2012, Facebook has over 900 million active users, more than half of them using Facebook on a mobile device. Users must register before using the site, after which they may create a personal profile, add other users as 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 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.