apps. they can update and use data, or the use of some apps use data. also mms (picture messages) use data. web browsing is actually one of the activities that uses very little. see if you can set somewhere to only udpate apps when on wifi. if you use BBM (the blackberry messenger thing) i believe it uses data. not positive, but i think so.
Backup apps will use data on a regular schedule. Facebook is a data hog and seems to access the web on its own for updates. In fact, and social app is likely to do automatic updates to bring you current notifications.