Facebook is a social networking service and website launched in February 2004, operated and privately owned by Facebook, Inc.  As of July 2011, Facebook has more than 800 million active users.  Users must register before using the site, after which they may create a personal profile, add other users as friends, and exchange messages, including automatic notifications when they update their profile.

For more information on Facebook, check out the site at http://www.facebook.com.