Facebook is a social networking service and website launched in February 2004, operated and privately owned by Facebook, Inc.
Users 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.
Facebook allows anyone who declares themselves to be at least 13 years old to become a registered user of the website.
As of January 2011, Facebook had more than 600 million active users and is the most used social network service by worldwide active useres.
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Facebook from Wikipedia