Instagram is a bad example. As far as we can see, Instagram has kept on growing and growing, and the ironic thing is that all those blog posts talking about how teens are leaving FB show them flocking to Instagram. So it was obviously a brilliant strategic move, and time will tell if it was a brilliant revenue generating move (I suspect Instagram will make buckets of money eventually, like YouTube does).

The key thing to look at to see if it's a bubble is IPOs. And as far as I can see, good companies are the ones IPOing successfully, and less stellar companies (Zynga, Groupon) are getting punished.