BB Bold :: Why Is 9900 Using 500mb Data

My daughter has just upgraded from a curve 8520 to a Bold9900 the problem is she has used over her limit of 500mb of data in under a month, she mosty uses facebook, but is not doing anything different from when she had her curve, we cannot understand how she is using so much data we have checked her bill and some days it is saying 30 mb of data which we thought was impossible, could anyone help me to sort out this problem, we have wifi in the house and have set her blackberry so she can use this at home but it stops her from sending or receiving messages and calls.