Simple: the bubble collapses when the acquiring companies no longer grow and succeed.
Facebook et al are doing well, but also have sky-high valuations. Current investors aren't satisfied with current income, they expect massive growth going forward.
I guess while I agree there is a bubble in startup valuations, I do not think Google, FB, and friends are at huge risk of going bankrupt or losing a substantial portion of their enterprise values.
The market is not about current, but about the future. Stockholders are banking on FB figuring out how to monetize all the users.
I do agree with you that companies today are mostly all making money which is very different from the 90s. Whether companies like FB can maintain growth to sustain their valuations is something that remains to be seen.