Used 3.5 gig in three days

This is not the first time I'm complaining about this. I loaded 2 gig data on my usb modem with an 8ta sim card on Tuesday 11 August. It lasted 3 hours. On Wednesday I loaded antoher 250 meg as I thought well I did some surfing on the internet looking for a pattern. The 250 meg was used up in less than an hour. Thursday morning 13 August I loaded another gig that was used up in 1h30 with only emails. How the hell is this possible??? I buy 2gig data from Vodacom for my cellphone a month, I am facebooking, doing emails and downloading. Not once did it last less than three weeks! I NEED RESPONSE ASAP!!!