Zuckerberg didn't try to develop a MySpace killer. He tried to develop a facebook[1] killer. thefacebook.com v0.1 wasn't built as a "social network", it was built as an online version of the college directory. That's why Zuckerberg won and the other thousand guys lost. His MVP was useful with only a few hundred users and a killer app with a few thousand. Everyone who tried to kill MySpace needed zillions of users and had no real way of getting them.

[1] http://en.wikipedia.org/wiki/Facebook_(directory)