If you use Internet on an iPhone app will AT&T charge you and data usage decrease?

Like if I use Facebook or something like that, will my data usage decrease an eventually get charged for it if I don't use wireless Internet? Just wanting to know.

Of course.
most apps use data, and you will be charged.

Yeah, any app that requires internet will use your data from your data plan until you use up everything you paid for. On Wi-Fi it doesn't count though, Wi-Fi doesn't affect your cellphone data. But whenever you have the "3G" at the top of the screen and you use internet whether it be in an app, browser, or anything like that, it WILL use up your data plan internet.

Using an app that uses the Internet, like Facebook, will use up your data unless you are on Wi-Fi at the time. Unless you are on an unlimited data plan, once you use up all your data for the month, you will be charged extra for additional data; the more extra data you use the more you will pay.

So to avoid problems, try not to use apps that use the Internet unless you're on Wi-Fi, especially apps that use a lot of data (anything that shows or plays videos or pictures or plays or downloads songs), also don't install/update apps unless you're on Wi-Fi.

You can tell if you're on Wi-Fi by looking at the top of the screen for the Wi-Fi symbol, it looks like this:

P.S. It's a little confusing when you say "will my data usage decrease". I know what you really meant, but when you say it that way, it literally means "will I use less data", which is the opposite.