Over the last six months, i've 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. Which go where gets kinda sticky, because it seems to primarily have to do with socio-economic class.
Boyd gets into some great discussion about the different ways social groups use social networking, even to the point of discussing how subaltern groups choose to modify their MySpace pages, while hegemonic groups prefer the unified look of Facebook (is this a difference in aesthetic tastes, or a more governing impulse to blend in / stand out?).
The essay isn't fully fleshed out, as Boyd suggests, but it does offer some very compelling thoughts to play with.