Rafe Needleman makes a great point about the new Facebook App Center. I firmly believe that standards-friendly HTML5 is where things are going, and I hope that the Facebook App Center will help push us in that direction.
So what can Facebook actually sell to users on the site? Non-platform-specific apps, or HTML5 apps. Facebook will take the customary 30% fee for app sales (Microsoft charges less, by the way) and may well make some money from this, but the real win for Facebook isnt retail sales of apps, its getting developers to build apps for the platform-agnostic standard of HTML5 instead of for proprietary operating systems.