One of my recent posts showed that the latest Facebook application increased "signaling" traffic significantly over the previous version (see "ALU: Signaling Issue with New Facebook" - here, with a comment from ALU providing additional explanation).
Facebook is not alone - some application could consume your monthly quota, even when in background. A Verizon support page warns about several Android applications that "might have serious negative effects on your device". One of the reasons to enter list is "Unexpected high data usage". The page has 13 applications, all games. Most are there since they keep the device from going to sleep mode, shortening batter life.
Some consume "a large amount of data while running in the background":
- Hill Climb Racing - A device left untouched with the app running could use as much as 6.4MB in 24 hours, or 190MB in a 30 day period
- Jail Escape - A device left untouched with the app running could use as much as 17MB in a 24 hour period, or more than half a gigabyte in 30 day period (removed from Google Play)