Why is my data usage so high? - 1

I have an iPhone 4 running iOS 6.1. I have a 500 MB data limit with Virgin Mobile. On about the 25 of February I got a text from them saying I had used up 450MB of my data. I checked today, which is the day my monthly bill ends (the 7th of every month) before I reset my data, and I had 792MB of cellular data received and about 85MB sent! I was absolutely floored at how much data I had used in about a week, maybe a week and a half. I definitely limited my Internet use during that period in hopes it would go down, and I only used it when I was at home where we have wifi. I'm almost always on wifi at school, and always on wifi at home, so I don't understand how so much data accumulated. Is there something in Settings that I need to turn on or off, or anything I need to do to prevent this from happening again? The apps I use mainly are Facebook, Instagram.
Please help, as this is very frustrating and costly!

Added (1). Also I reset my data at 8:30 this morning, and if I check it now, I have already 97MB of data (9.5 received and 1.4 sent) used! I used the Internet for about a half hour today for listening to music on YouTube. What is going on?

Youtube
automatic updates
facebook
are all high data users.
get a widget to turn off your 3G when you are not using it.

Listening to music on youtube. Gee, what do you think it could be? Anything streaming is obviously alot of data.

On my android I have an app called onavo that's tracks what is using 3g data and how much, maybe their is Simmilar on iphone? Try searching data tracker, then you can see what is eating all your data and unninstall it or turn off its permissions depending on what it is