Aleks forwarded this article by Danah Boyd:

Over the last six months, I [Boyd] have noticed an increasing number of press articles about how high school teens are leaving MySpace for Facebook. That’s only partially true. There is indeed a change taking place, but it’s not a shift so much as a fragmentation. Until recently, American teenagers were flocking to MySpace. The picture is now being blurred. Some teens are flocking to MySpace. And some teens are flocking to Facebook. Who goes where gets kinda sticky… probably because it seems to primarily have to do with socio-economic class.

Much discussion follows. No numbers but I imagine somebody’s done that too.