If Facebook can successfully buy out every other significant social network, then they can maintain market leadership. And their stock will stay pretty high.
Does anti-trust ever become an issue in that case? What about availability of cash (they just spent 35% of theirs)? What about properly valuing users?
Those are good questions, not really sure. I think in the very long run, Facebook's continued success depends on if they can quash net neutrality and make it very difficult or expensive for social networking startups to gain traction.