Facebook is a social networking website that is operated and privately owned by Facebook, Inc.Since September 2006, anyone over the age of 13 with a valid e-mail address can become a Facebook user. Facebook's target audience is more for an adult demographic than a youth demographic.Users can add friends and send them messages, and update their personal profiles to notify friends about themselves. Additionally, users can join networks organized by workplace, school, or college. The website's name stems from the colloquial name of books given to students at the start of the academic year by university administrations in the US with the intention of helping students to get to know each other better.

Facebook was founded by Mark Zuckerberg with his college roommates and fellow computer science students Eduardo Saverin, Dustin Moskovitz and Chris Hughes. The website'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 later expanded further to include (potentially) any university student, then high school students, and, finally, to anyone aged 13 and over. The website currently has more than 400 million active users worldwide.

The original concept for Facebook was borrowed from a product produced by Zuckerberg's prep school Phillips Exeter Academy which for decades published and distributed a printed manual of all students and faculty, unofficially called the "face book".

Facebook has met with some controversy. It has been blocked intermittently in several countries including Syria, China, Vietnam, and Iran. 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 settled a lawsuit regarding claims over source code and intellectual property.The site has also been involved in controversy over the sale of fans and friends.

A January 2009 Compete.com study ranked Facebook as the most used social network by worldwide monthly active users, followed by MySpace.Entertainment Weekly put it on its end-of-the-decade '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?"

There have recently been reports of Facebook proposing an initial public offering (IPO), i.e. issue equity shares as stock to investors. However, Zuckerberg stresses that it will not be for a few more years, and the company is in no need of additional capital. Also, some analysts fear the Facebook IPO might be a particularly weak one.