Launched in 2003, Thefacebook.com was an evolution of Mark Zuckerberg’s earlier site facesmash – a hot or not style site that allowed Harvard students to vote on which of their classmates were hotter.
Initially restricted to Harvard students, within a month of launch more than half of Harvard’s undergrads were on thefacebook.com. In March 2004, Facebook expanded to Columbia, Stanford and Yale. The rest of the Ivy league and most universities and colleges in the US and Canada quickly followed.
By 2009 Facebook was ranked the most used social networking service worldwide. For more info, see the Wikipedia entry.