Facebook (sometimes shortened to FB) is a social networking service and website started in February 2004. It was built by Mark Zuckerberg. It is owned by Facebook, Inc. , Facebook has over one billion active users. Users may make a personal profile, add other users as friends, and send messages. Facebook users must register before using the site. The name of the service comes from the name for the book given to students at the start of the school year by some universities in the United States. These books help students get to know each other better. Facebook allows any users who are at least 13 years old to become users of the website.