It will, of course, be harder to grow a privacy-enabled service than a promiscuous one. It's harder to establish the viral loop. But that doesn't mean it could not happen, and it is probably too soon to tell if or when it will.
Facebook will look fine for a while, but the damage to their brand is permanent. People don't jump ship without motive, means, and opportunity, but now the motive is firmly in place for a lot of users. Wait and see what happens if and when someone invents the means and offers the opportunity.
Until a service exists and I can migrate there with all of my friends, I need Facebook to fill that role. Yeah, I could just restrict myself to those who seek me out (or I seek out), but that would likely mean discarding some people who might have been great lifelong friends.