you are talking about the killer app, approach. If privacy, security are not killer apps what is?
Facebook has image storage limits, although high, there might be some room there.
Killer Personal Analytics could be amazing for some people like microbrands and celebs, but no one else would care.
Better game platforms ( kabam and zynga are attacking this )
Better Data storage might be an interesting one, google apps and dropbox have shown strong trends here.
App.net is looking into a lot of these approaches.
Another big one would be 4th party delegated identity, or anonymous micro payments.
In the end Facebook just has not abused users enough for them to know what can go wrong, if this carries on for decades. In the end people trust Facebook to do what is the best compromise.