Looking back since 2008 or so, with the possible exception of groupon, I can't think of a single $5+ billion dollar web 2.0 company that has crashed and burned.

A significant number of these web 2.0 companies seem to have defied the ‘crash and burn’ that characterized the first boom. Pets.com and Webvan were pretty much failures from inception, but sites like Pinterest and Airbnb keep getting more successful and more valuable.