If people start getting racked with real-world consequences because of things they put on Facebook, like lost work opportunities, legal ramifications, harm to their reputation, and so forth to a point where the average person (non-techies) becomes concerned about privacy to the point of avoiding social networks like Facebook entirely then Facebook's growth model is going to be in jeopardy. As for what will replace Facebook in the event that this happens - probably closed networks along the lines of LinkedIn.

I'd say it'll take a few years of things going the way they are now before the public at large starts taking their privacy a little more seriously.