Facebook is an online social networking service headquartered in Menlo Park, California. Its name comes from a colloquialism for the directory given to students at some American universities. Facebook was founded on February 4, 2004, by Mark Zuckerberg with his college roommates and fellow Harvard University students Eduardo Saverin, Andrew McCollum, Dustin Moskovitz and Chris Hughes. The founders had initially limited the website's membership to Harvard students, but later expanded it to colleges in the Boston area, the Ivy League, and Stanford University. It gradually added support for students at various other universities and later to their high-school students. Facebook now allows anyone who claims to be at least 13 years old worldwide to become a registered user of the website, although proof is not required.
After registering to use the site, users may create a personal profile, add other users as friends, exchange messages, post status updates and photos, and receive notifications when others update their profiles. 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 "Close Friends". Facebook had over one billion active users as of September 2012, of which approximately 9% were fake. By that point, Facebook was adding about half a petabyte of data every 24 hours, amounting to about 180 petabytes per year. Due to the large volume of data collected about users, the service's privacy policies have faced scrutiny, among other criticisms. Facebook, Inc. held its initial public offering in February 2012 and began selling stock to the public three months later, reaching a peak market capitalization of $104 billion.