According to Wikipedia and its Info page, Facebook was launched exactly six years ago, on February 4, 2004. Back then it was called "Thefacebook," and it was originally located at thefacebook.com, but it's definitely the same project.
Founded by Mark Zuckerberg, together with his college roomies Eduardo Saverin, Dustin Moskovitz and Chris Hughes, Facebook was originally intended to be a network only for Harvard students, but was later expanded to other universities and finally to everyone.
Intelligent design, a lot of smart business decisions and gradual expansion made Facebook what it is today: by far, the biggest social network on the Internet.