I've been having the exact same problem. I live in an area that does not have LTE coverage, so I turned that off. Usually, I never come close to even hitting 65% of my 2GB data plan. However, upon getting my iPhone 5, I not only exceeded the plan, but I used an additional gigabyte. I called AT&T, and the rep I spoke to actually helped me a lot. He listed a few tips that will cut down data usage:
-Turn off "Anonymous Data Collection" in Settings>General>About>Diagnostics and Usage>"Don't Send"
-Turn off "Location Services" in Settings>Privacy. There are ON/OFF switches for apps that request your location, and the only apps that I allowed to use location services were obvious essentials like Siri, Maps, and Weather.
-Turn off LTE (Settings>General>Cellular) if you are in an area that does not provide LTE coverage.
-For those with iTunes Match, make sure that you go to Settings>Music and turn OFF "use cellular data" in the iTunes Match settings section.
If this is not making a noticeable difference, turn off your data and call AT&T. Yes, it is inconvenient to do so, but AT&T definitely won't be refunding you or feeling any sympathy because your iPhone 5 is a data hog.