|Type of site||Social networking service|
|Available language(s)||Multilingual (147)|
|Users||1 billion (active October 2012)|
|Written in||C++, PHP and D language|
|Launched||February 4, 2004|
|Revenue||Increase $5.1 billion (2012)|
|Alexa rank||Steady 2 (December 2013[update])|
Facebook (sometimes shortened to FB) is a social networking service and website started in February 2004. It was built by Mark Zuckerberg. It is owned by Facebook, Inc.As of September 2012[update], Facebook has over one billion active users. Users may make a personal profile, add other users as friends, and send messages. Facebook users must register before using the site. The name of the service comes from the name for the book given to students at the start of the school year by some universities in the United States. These books help students get to know each other better. Facebook allows any users who are at least 13 years old to become users of the website.
Facebook was started by Mark Zuckerberg with his college roommates and fellow computer science students Eduardo Saverin, Dustin Moskovitz and Chris Hughes. The website's membership was only for Harvard students at first. Later it included other colleges in the Boston area, the Ivy League, and Stanford University. It eventually opened for students at other universities. After that, it opened to high school students, and, finally, to anyone aged 13 and over. Based on ConsumersReports.org in May 2011, there are 7.5 million children under 13 with accounts. This breaks the website's rules.
A January 2009 Compete.com study ranked Facebook as the most used social networking service by worldwide monthly active users. Entertainment Weekly put the site on its end-of-the-decade "best-of" list. It said, "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 had 138.9 million monthly different U.S. visitors in May 2011. According to Social Media Today, in April 2010 about 41.6% of the U.S. population had a Facebook account. Facebook's growth started to slow down in some areas. The site lost 7 million active users in the United States and Canada in May 2011 relative to previous statistics.
Facebook has been involved in many controversies over privacy. Some of these controversies have been about people being able to see personal information that other people post, and others are about companies and advertisers being able to see users' personal information.
Research published in the journal PLOS ONE has shown that Facebook may be responsible for spreading unhappiness through society as well as keeping people connected. Scientists found that the more time people spent on Facebook over a two-week period, the worse they subsequently felt. "On the surface, Facebook provides an invaluable resource for fulfilling the basic human need for social connection. Rather than enhancing well-being, however, these findings suggest that Facebook may undermine it." 
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