How about charging users $1 per year for the mobile app. If this were to happen it would either make people stop using mobile (unlikely) and they will resort to using the website (which is good for FB) or make a billion per year in revenue. Plus this would change the dynamics of free apps forever. I think it would be interesting for the mobile app ecosystem as a whole... more people would charge for apps.
I have a better idea. How about charging users to get access to someone else's account?
How about charging users to forcibly become friends with someone who has rejected them? It could even be a recurring monthly fee to renew the friendship.
That is the dumbest thing I have ever heard.
... It's private photos so you can personally limit its exposure. When that trust breaks, Facebook won't last long after.