This is a good point to go into. At this point Facebook essentially needs to monetize their userbase to an extraordinary degree. That's the debt they've incurred due to the pricing of their IPO. That will likely force them to annoy their users with more ads, etc.

However, consider an upstart with nothing to prove. If they can put together a facebook competitive technology stack at a lower cost they can actually monetize less than facebook, and steal users that way. The fact that they would potentially make, say, half or a tenth as much revenue per user is irrelevant, because it would be their revenue vs. facebook's revenue.