Iphone and blackberry data usage?

I had a blackberry and was on an o2 contract that had 200mb of internet. I have now got an iphone, i got my blackberry internet removed so now i only have 100mb but i seem to have used that up so quickly, in a week i've just about used all of it up yet i'm using my iphone internet just as much as i did my blackberry! I need to get more internet but not sure how much i will need, 250mb or 500mb? I don't use it that often, just check facebook and twitter lots, and occasionally instagram and google every so often!

250 is good