In 1999 boom, Nasdaq P/E reached 200, while general stock market P/E was 'only' 34. 6 times the difference. This time it is around 24 and general market is at about 19. Just 25% the difference. So i think there is really nothing to be worried about. There are few extreme valuations, and when those correct (Zynga, Groupon, ...) it doesn't create a domino effect.<p>Even Snapchat doesn't sound so stupid - it may never make any revenue, but having such a crowd of loyal users can bring a lot of cash to many companies who have already figured out their monetisation (Apple, Google, etc.) so they will be just buying a huge market. And they have a lot of cash to pay for what they buy, and sometimes they do actually buy. So people investing in Snapchat on these seemingly extreme valuations are likely not that stupid, or subversive.