Facebook is

Facebook is a social networking service and website launched in February 2004 that is owned and operated by Facebook, Inc..  In May 2012, Facebook has more than 900 million active users, more than half of them using mobile devices.  Users must register before using this site. After that, the user can create a personal profile, add other users as friends and exchange messages, including automatic notifications when they update their profile. Additionally, users can join a group of users with the same interests, sorted by workplace, school or college, or other characteristics, and categorize their friends into lists such as "Colleague" or "Close Friends". The service name is derived from the name of books given to students in the first academic year by some university administrations in the United States with the goal of helping students get to know one another. Facebook allows anyone at least 13 years old to be a registered user of this site. 

Facebook was founded by Mark Zuckerberg with his roommates and fellow computer science students Eduardo Saverin, Dustin Moskovitz and Chris Hughes.  Membership website was originally limited to Harvard students only, then expanded to other colleges in the Boston, Ivy League, and Stanford University . This site is slowly opening up to students at other universities before opening to high school students, and eventually to all persons at least 13 years. Even so, according to the Consumer Reports survey in May 2011, there were 7.5 million children under the age of 13 years who have Facebook accounts and 5 million under 10 years old, so it violates the site's terms of service.

In January 2009 Compete.com study puts Facebook as a social networking service most widely used by number of monthly active users worldwide.  Entertainment Weekly put it on my list of "best" end of the decade with the comment, "How can we stalk our ex-lover , remember our co-workers birthday, teasing us and playing Scrabulous before Facebook was created? " Quantcast estimates Facebook has 138.9 million monthly visitors in the U.S. in May 2011.  According to Social Media Today in April 2010, approximately 41.6% of the U.S. population has a Facebook account. Even so, the Facebook market growth started to fall in some areas with the loss of 7 million active users in the United States and Canada in May 2011.

source : http://id.wikipedia.org/wiki/Facebook

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