How does Android determine which apps are "social networking" apps for data usage purposes?
Your carrier determine a social network by his hostname or IP as said in comment, not with a specific application. But there is sometimes where this feature doesn't work with apps ( some apps use proxy, don't know if this is the case for facebook or twitter... ). 
An other case is this only work on the carrier's website (if they provide social network functions directly on their site), but it would be said on your carrier's offer.

Anyway this is only determined by the IP or hostname. You can't do that by designate an app with a certain programming function...