People keep saying we're in a bubble and yet seven years later, since 2008, I can only think of three major VC implosions off the top of my head: fab.com, zynga, and groupon. Fab was a train-wreck that anyone with a pulse could have seen coming. Zynga... way too dependent on Facebook and a fad. Groupon is the better of the three, only seeing its valuation fall from $30 billion to just $4 billion - but still well over $1 billion. These valuations, as lofty as they seem, are not falling. It's kinda weird, but not surprising considering how much wealth is circulating in the silicon valley. Then you look at twitter and it held up very well after its IPO despite its very high valuation.
The problem with using the past to predict the future is that there are often subtleties that can produce wildly diverging outcomes. These app & web 2.0 companies seem to be doing something right to avoid the valuation chopping block that afflicted so many of the earlier internet startups. web 2.0 companies retain value better than a CD lol
One reason why app companies, as opposed to sites that that sell physical stuff, are doing so well is that they have better profit margins and can use network effects for added growth.