With a big enough user-base, they'll find a way. Occasional contextual adverts added to your timeline seems the obvious start.
I think it's time we stop taking that for the gospel and admit that it's actually quite possible for a company to get a huge user base and still fail to make money permanently.
That wouldn't exactly be comforting if it was my money on the table.
But there has to be an inflection point somewhere. Look at youtube, they will never be in the black (taken on their own).
So either they turn a profit, and this piece of wisdom is strongly reinforced. Or they go under, and entrepreneurs and VCs lose a certain glint in their eye. Or they get bought... and probably no one's mind is changed.
Just like Facebook, right!?
It doesn't matter how big either of those numbers are, as long as the equations stand. Most argue that the equation in facebook's case fails.