They have cash (quite a bit, post-IPO) and revenue. Companies with cash and revenue don't collapse, even if their stock is overvalued. Myspace didn't "collapse" either, though ultimately it lost in the market to a better competitor.

It's possible Facebook will ultimately lose too, but it won't be because of their stock value or their being "overhyped". Hell, statistically it's much more likely that they'll be beaten by a much, much smaller company (though I guess G+ might do it too).