We might be in a bubble. But that bubble will continue to inflate as long as people are screaming Bubble! Bubble!! The danger moment appears when all those predictions appear false and everyone start to believe, maybe this time, for some reason, exponential growth will continue forever.
The moment when everyone shut up and start to join the bandwagon is when the bubble bursts. <-- That's from my experience.
Your last sentence is very astute, and resonates with me personally. I thought valuations were insane in the late 90s but ultimately gave in. I think that was less than 6 months before the crash!
Regarding exponential growth. We need to think carefully what kind of efficiency improvements can SUSTAIN exponential growth. The adoption of the Internet to the mainstream was a major event. I guess people might be betting that mobile/tablet is another one. I thought this was already priced in about 2 years ago. I can't think of what has changed in fundamentals that justifies the current situation (except for inflation perhaps).
The moment when everyone shut up and start to join the bandwagon is when the bubble bursts. <-- That's from my experience.