Vodacom exorbitant data accounts

I have crazy data bills every month.  I thought it was because I downgraded my contract and only get 250megs a month.  I bought an extra 750megs and still my data bill was high.  I then decided that wifi was the way to go. I got wifi at home,  I have wifi at work. My data account is over R2000 this month.! Low and behold...I got a call from Vodacom to sell me a contact for a mobile wifi device,  which I signed up for.  I now even have wifi in my car!  I turn my mobile data off.  Then I got a message one day to say my data is depleted,  even though I hardly use it.  My mobile wifi device which is supposed to give me 2gigs of data a month is also depleted!  I only use data for IM Facebook and twitter and only to receive messages in the car.  How the heck do I manage to run out of data when I have wifi everywhere!  I logged a call with Vodacom who have been investigating for a week already with no word.