I don't know if Facebook traffic is tanking or not but I do know that I haven't been visiting Facebook as often as before and noticing more spam-like notifications from Facebook in my mailbox. And it's pretty difficult to find even a handful of Facebook apps I can point to as being really useful.
I doubt MySpace will fare better with their upcoming upgrade. Silos just don't make great application platform and mashups are just windows bound to be closed. Something has to give here and now.