A recent Newsweek article describes research by Danah Boyd of the Berkeley School of Information, and quotes her description of who goes where:

A few weeks ago, Boyd—who has done extensive ethnographic work on
online behavior, blog-posted an essay tentatively sharing her
(admittedly nonscientific) findings after months of interviews, field
observations and profile analysis. Generally, she contended, "The
goodie two shoes, jocks, athletes and other ‘good’ kids are now going
to Facebook. These kids tend to come from families who emphasize
education and going to college." MySpace is still home for "kids whose
parents didn’t go to college, who are expected to get a job when they
finish high school."

She writes later,  "The division around MySpace and Facebook is just another way in which technology is mirroring societal values."

Something to talk about this fall….