A new social network for private networking will popup to fill the void facebook is leaving, just as facebook originally filled the need when myspace was at its peak.
I don't understand why facebook can't just keep doing what made it popular. The fact that it was a closed network for only real people you knew, not random internet "friends". Why follow in the footsteps of twitter when you already have something useful. I stopped using facebook once my friends list grew into the hundreds, trying to open it up even more is probably going to result in less usage.