Alternatively, Facebook could be out of fashion in 2 years. Don't forget that we work in a fashion industry. It wouldn't even be worth its last post-money valuation.
So the expected value of Facebook lies somewhere in between and that expected value is radically short of management's dreams. They don't need cash and they seem to be executing well. This is not a recipe for strategic acquisition.
I think Facebook will remain private and independent for as long as it can. If they are on a path to success and need more cash, they'll IPO (remember those?). If they keep their user base, but can't scale their monetization, they'll be bought by somebody who can (a search engine). The worst case isn't pretty because they don't have the backstop that has been supporting MySpace through its troubles (music).