The Internet ethnographer danah boyd takes a look at class divisions in the online world. A pattern emerges: The "good" kids tend to go to Facebook, the "bad" ones to MySpace.
Facebook was framed as being about college. This was what was in the press. This was what college students said. Facebook is what the college kids did. Not surprisingly, college-bound high schoolers desperately wanted in.
In addition to the college framing, the press coverage of MySpace as dangerous and sketchy alienated "good" kids. Facebook seemed to provide an ideal alternative. Parents weren't nearly as terrified of Facebook because it seemed "safe" thanks to the network-driven structure. (Of course, I've seen more half-naked, drink-carrying high school students on Facebook than on MySpace, but we won't go there.)