According to Zynga's IPO they get $0.32/MAU/month. Obviously this fluctuates considerably depending on how aggressive your strategy is for extracting cash from your players (or users). Zynga is very aggressive but also has a frightfully high burn rate. That's why they have to release fresh games regularly.
The other missing information is whether that number is per unique user across multiple games or per user per game.
If your game allows users to purchase things with Facebook credits, then your take will depend significantly on your ability to sell things to the users. If you're relying on advertising alone, it will take a lot more.