Exactly this is happening to me as well, since I've upgraded to Lumia 820 my data usage just went so high over the roof !
I have only 2 mailboxes + facebook setup and most of the day phone uses WIfi at home, the o2 chap suggested to swich off mobile data while in wifi range and i argued that as this is not the reason why i bought a smartphone to go around and look if I'm within WiFi range, however i did started to switch off mobile data while at home just to find out that when i go out and switch data back on, this got registered as >100mb + of data usage in that minute even when the phone was on wifi with data off since last afternoon !!!!
I have now argued that with o2 and hoping to resolve the issue.
I should add that i had previously an nokia X6 with the same mailboxes and facebook setup and i was not even getting 400MB+ of usage out of my 1gb allowance.
there is clearly something wrong!
