I have a iphone 4s on Vodafone's pay monthly.
While I am using wifi, my Cellular is being used and it's going up every 5 minutes by KB not MB.
But as you know KB will eventually turn into MB and one day this will have me going over my Data limit (250mb).
I've only tried Double clicking my home button to see if there are any background apps.
I can't think of anything else to try to find out what is using my Cellular data!
Ps: I have push notifications enabled on Facebook/Instagram/Twitter etc but even when i'm not recieving them, something is taking my mb.