Why is my moblie data being used up so quickly?

I have an iPhone 4s that is on a 500mb a month contract and it seems to be being used up very quickly.
For example the start of the month when I get new data, texts and calls is on the 3rd and on the 9th I check how much data I have used by calling a number that Vodafone provided me with. I have used 230mb.
I don't know why this is happening because I don't Use the Internet on my phone very much just to check Facebook and look up something once in a while and iMessage. I never watch videos or stream music.
When I'm at my house or at work I connect to the wireless and use that instead of using the moblie data. I do watch videos at this time but I make sure that I'm connected to the wireless. Why am I usin up so much moblie data?

Check that you don't have apps that are updating themselves that is a major problem with data usage that you don't see.