On the other hand, online user bases are notoriously mercurial. Facebook's valuation implies an assumption that Facebook won't fall victim to the forces that have claimed every social site to come before it.
That's a pretty gutsy assumption, considering Facebook could plausibly be seriously damaged by something as simple as kids deciding they don't want to hang out at the same place their parents do.
You do realize you could say the same thing about pretty much every company in existence. No-one ever stays on top forever, it's a question of how long you can stay on top for and how much profit you can generate over that period.
That's a pretty gutsy assumption, considering Facebook could plausibly be seriously damaged by something as simple as kids deciding they don't want to hang out at the same place their parents do.