Facebook is one of the few companies in the world with unbounded potential. It's a cultural phenomenon that has changed the way people use the internet and, more importantly, has changed the way people interact with their real friends. Very few companies change culture and daily interaction like this.

Is it worth $33b today? Not sure. Could it be worth $200b someday? Could Facebook be bigger than Microsoft? Bigger than Google? Bigger than Exxon? Maybe. I can't think of any other company for which "maybe" is a reasonable answer.

That's why investors value it so highly.