Facebook won because it was from Harvard, to put it simply.

FACEBOOK: LIKE? | More Intelligent Life: And what is Facebook, anyway? The most obvious point of historical comparison is the social networks that preceded it. First there was Friendster, the flirt-and-forget site of the first half of the 2000s. Then everyone dumped Friendster for MySpace, and MySpace was bought by News Corp for $580m. Its value soared to $12 billion, and the received wisdom was that MySpace would take over the world. Then it didn’t, and News Corp sold it for $35m, because someone else had finally got social networking right. Started by Mark Zuckerberg in 2004, Facebook went from a Harvard dorm room to the rest of teenage America’s bedrooms to hundreds of millions of people all around the world—even parents and grandparents. Along the way, Facebook has fuelled revolutions in the Middle East, and inspired an Oscar-winning movie. Other social networks can only try to build out from the few niches it hasn’t already filled. Facebook is the undisputed champion of the world.