1. Facebook - 2004
Facebook was famously concocted by the crafty (though some would say shifty) Mark Zuckerberg in his Harvard dorm room in 2003. The site officially launched in 2004 - then called "The Facebook" - as a bare-bones social network for colleges and universities. Needless to say, Facebook has expanded in the past seven years to be one of the most visited sites on the Web. It currently has over 800 million active users.