Facebook is a social networking website that is operated and privately owned by Facebook, Inc. Users can join networks organized by city, workplace, school, and region. People can also add friends and send them messages, and update their personal profiles to notify friends about themselves. The website's name stems from the colloquial name of a book given to incoming students at Zuckerberg's high school alma mater, Phillips Exeter Academy. The book shows the faces and names of the school's students and faculty.

Mark Zuckerberg founded Facebook with fellow computer science major students and his roommates Dustin Moskovitz and Chris Hughes while he was a student at Harvard University. Website membership was initially limited to Harvard students, but was expanded to other colleges in the Boston area, the Ivy League, and Stanford University. It later expanded further to include any university student, then high school students, and, finally, to anyone aged 13 and over. The website currently has more than 250 million active users worldwide.

Facebook has met with some controversy over the past few years. It has been blocked intermittently in several countries including Syria, China and Iran, although Iran later unblocked Facebook in 2009. It has also been banned at many places of work to discourage employees from wasting time using the service. Privacy has also been an issue, and it has been compromised several times. Facebook is also facing several lawsuits from a number of Zuckerberg's former classmates, who claim that Facebook had stolen their source code and other intellectual property.

A February 2009 Compete.com study has ranked Facebook as the most used social network by worldwide monthly active users, followed by MySpace.

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