3.35GB (Not MB) Data gone - after a few hours

I have a 5GB DATA contract which I was supposed to get on the 1st but only got on the 2nd as Vodacom was DOWN THE WHOLE DAY. I went onto FB on the 2nd and it was eventually working. My 5GB bundle lasts me a whole month being on Facebook daily & playing games daily. To my horror after a few hours I noticed, because I always check, that I only had 1.65GB left. I never download any movies or anything else & when I am done on FB I disconnect my modem. Calling the call centre,. An absolute nightmare. I was told they would look into it & it would take 7 to 14 days. I got a call from Zanele 4/8 & was told she is escalating the matter. FB comment to my complaint \If you have an android device you should be able to see what applications used your data. When using wifi it is advisable to turn your mobile data off. If the wifi signal strength gets weak it will switch to mobile data. ^AF". I use this on my computer only. Also"