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I think IPOs are always iffy. There hasn't been anytime for the stock's natural valuation level to have been determined in the marketplace. It is a hope, based on a limited information and history.
It may just be my natural skepticism, but I just don't see Facebook as an innovative company that will indefinitely grow. I see increased privacy concerns, both from an individual and legislative standpoint, as a potential limiting factor. The question, as I see it, is whether Facebook has become, or in the future can become, a necessity of modern life. You have some movement towards that in sites that require you to log in using Facebook (turntable.fm comes to mind). So you are force to engage with Facebook, whether you want to or not. Of course, that can result in unintended blowback. Speaking for myself, I now only use Facebook to log in to sites that make me use it. I don't accept friend requests because I don't want to be forced to share all of my friends information with countless entities.
The other possibility is that something new comes along that we haven't considered. Look at MySpace; it was the Big Dog for a period, and while it has experienced a bit of a resurgence lately, I doubt many people would consider it really important that one have a MySpace account. Google+, while not yet a true competitor, may evolve into one over time. All it takes is one new "thing" to come around, whatever it may be, and give people a better alternative to Facebook.