I think the primary driver in the exodus from MySpace to Facebook was the Feed. The experience of Facebook is just so much more "social" because of it. Other things like design/perceived professionalism/etc didn't hurt either, of course.
Yeah, I agree that those were important, maybe more important factors too. But I know alot of my non-tech friends made the move largely because they were sick of myspace spammers constantly contacting them.
Oh, interesting. I was thinking of "privacy" in terms of not letting their personal information get out; but there's certainly that issue, too!