There are two problems with facebook's P/E. The first is that it's questionable whether they can grow revenue enough to justify such a figure. They can't grow their userbase by a factor of 10 because there aren't enough humans on Earth. And they have a huge roadblock in terms of further monetizing individual users. Ultimately investing in facebook isn't a bet that they'll keep doing what they're doing now (as it is with amazon or apple), it's a bet that they are going to fundamentally change everything about how they advertise and somehow garner a huge increase in per user revenues.

Second, facebook stock is already priced at a level where it's expected they'll be taking in a huge percentage of total worldwide ad revenues. Where can it go up from there? There isn't much reasonable headroom there, which makes it a very poor stock to hold on to.