Myspace is irrelevant, but not quite dead yet. Facebook like IBM is going to stick around in one form or another for a long time. Their real risk is maximizing profit may not be the same as maintaining dominance, so they may slowly kill off their service over 10+ profitable followed by a slow decline.

And really Facebooks main problem is they have so many users they can't really grow revenue by adding more. Which means revenue growth is generally going to drive people away.