The approach to social networking they've taken simply isn't a particularly profitable approach, the profitable approach hasn't been invented yet. Eventually, people will realize that, and while they're running and begging venture capitalists for another $100 million just to keep the lights on in their server farms, someone else is gonna build the next big thing, and Facebook will go the way of Friendster and Myspace. If we've learned anything, social networks are fickle, and it doesn't take long for a trickle of users to become a flood.
Zuckerburg won't ever starve, and the company probably won't ever fold (heck, even AOL is still around), but the idea that his wealth and any perceived power is in any way stable just doesn't check in with reality.