Facebook is a social networking service launched on February 4, 2004. It was founded by Mark Zuckerberg with his college roommates and fellow Harvard University student Eduardo Saverin. The website's membership was initially limited by the founders to Harvard students. In 2005, Facebook expanded to twenty-one universities in the United Kingdom and others around the world. Facebook was then opened on September 26, 2006 to everyone aged 13 and older with a valid e-mail address. Currently, in 2015, there are 1,550 million Facebook users worldwide.