Personally, I think it's unwise to bet against Zuckerberg, and here's why I think so. He has struggled with the Peter Principle, and IMHO beat it, which I gotta say is not an easy feat. But he also bet big on things most wouldn't.

FB has gone though crazy struggles before, but if my understanding of both PG and YC is correct, you bet on the people, not the product. I have a feeling that if FB Inc has to reinvent itself, it will manage to do so.