Wow, talk about over-analyzing the situation
True, the sites have very different audiences because they were built with different audiences in mind initially, and grew out from those audiences. I can't speak for Myspace (while I did set up an account there, I never use it since most of the invites I get are spam), but for Facebook, since it started out as a site solely for college students, it grew out from that audience to include college grads who were friends with Facebook users who graduated, and built up from there.
But the analysis of the differences in home pages is just silly now that we have those custom apps for Facebook.