Samsung galaxy S3 continuously downloading on gprs

My mother in laws phone continuously downloads from GPRS, seemingly even when wifi is available. Usually the downloads are very small and in multiples over a short period. occasionally they are much larger, up to 50MB. The records from the provider indicate that some of these downloads are from Facebook and youtube, however she has not used these apps or logged in. The downloads are occurring so often that she is getting warnings that she will exceed her quota (200MB/month). She has set apps so they don't update automatically. Also she syncs three gmail accounts to the phone. Does anyone know what these frequent downloads may be, and also if it is possible to limit them to only when wifi is available. Thanks