My go to example for this is Facebook's IPO. Their valuation was at the time at $50/user. Can they extract that much lifetime value out of every user they have? It seems rather high to me.
I don't believe there is an industry-wide bubble going on. But it seems to me that if you hit the right keywords (social, sharing, advertising), you will be valued at the top of the range, not the bottom. Moreover, I believe these particular valuations will get market corrected sooner or later.