When IPhone app isn't launced is my data plan still being used?

I have apps like Facebook, Twitter, & Photobucket that I keep logged in so as soon as I launch the app it's where I want it. Is the data plan still in use?

What about the iphone Mail app that's kept on as well?

I was told by an ATT worker that if they are just open and you aren't doing anything on them, that no data is being used.

Just check your data plan after an hour to see if it went up, it shouldn't…