Fred's article highlights the risk to Facebook if there's a bubble and it bursts. As well as looking at some serious security/privacy challenges at the platform level and a hard-to-sustain engineering pace, they also need to worry about how long irrational exuberance continue. If they IPO at $10-$20B, that'll be a sign of air leaking out of the bubble. If they hit the wall before IPO, or crash badly in the first six months out of the gate, it'll be a lot more dramatic.