Luckily it's guaranteed to die , because its foundation is totally unsustainable.
For every long-lost chum who reaches out to me on Facebook, there's a guy who beat me up on a weekly basis through the whole seventh grade but now wants to be my buddy; or the crazy person who was fun in college but is now kind of sad; or the creepy ex-co-worker who I'd cross the street to avoid but who now wants to know, "Am I your friend?" yes or no, this instant, please.
It's socially awkward to refuse to add someone to your friends list -- but removing someone from your friend-list is practically a declaration of war. The least-awkward way to get back to a friends list with nothing but friends on it is to reboot: create a new identity on a new system.
That's why I don't worry about Facebook taking over the net. As more users flock to it, the chances that the person who precipitates your exodus will find you increases.
People really need to rethink their assumptions when they get all worked up about Facebook, Myspace, or whichever social site has the biggest/fastest-growing community at any given time. Those communities are transient communities, like nightclub communities. Building Facebook apps can be a good idea, but it's an idea with an inherently limited shelf life.