Not all Android users have an unlimited data plan. Monitoring data use is a pain, but Android Jelly Bean makes it a little easier. Go to Settings > Data usage to see how much data each of your apps is using. You can program your phone to warn you when you're approaching a high amount of data use and stop you altogether when you reach a certain limit. Users of the Froyo, Gingerbread and Honeycomb versions of Android (2.2 through 3.2) can download Onavo Count, a free app that serves the same purpose but also lets you set data restrictions on individual apps.