Facebook is a social networking service and Web site launched in February 2004, operated and privately owned by Facebook, Inc. As of July 2011, Facebook has more than 800 million active users. 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 "Really Good 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.

Facebook was founded by Mark Zuckerberg with his college roommates and fellow computer science students Eduardo Saverin, Dustin Moskovitz and Chris Hughes. The Web site's membership was initially limited by the founders to Harvard students, but was expanded to other colleges in the Boston area, the Ivy League, and Stanford University. It gradually added support for students at various other universities before opening to high school students, and eventually to anyone aged 13 and over. However, based on ConsumersReports.org on May 2011, there are 7.5 million children under 13 with accounts, violating the site's terms of service.

A January 2009 Compete.com study ranked Facebook as the most used social networking service by worldwide monthly active users, followed by MySpace. Entertainment Weekly included the site on its end-of-thedecade "best-of" list, saying, "How on earth did we stalk our exes, remember our co-workers' birthdays, bug our friends, and play a rousing game of Scrabulous before Facebook?" Quantcast estimates Facebook has 138.9 million monthly unique U.S. visitors in May 2011. According to Social Media Today, in April 2010 an estimated 41.6% of the U.S. population had a Facebook account. Nevertheless, Facebook's Market growth started to stall in some regions, with the site losing 7 million active users in the United States and Canada in May 2011.

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