How does Android determine which apps are "social networking" apps for data usage purposes? My phone carrier (Bell Canada) says that in addition to my maximum data limit, I have "unlimited social network" usage.

I'm wondering which apps count as social networking apps? As an Android developer, I'm also interested in how I might designate one of my apps as a social networking app.

(Let me know if this would be better off on programmers.stackexchange or stackoverflow...)
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...