Data usage

You could change email to fetch instead of push. Also, you can go to setting, then cellular, scroll down and it will show what apps are using amounts of cell data.

 

If you're on iOS7, you can see exactly how much 3G/4G/LTE data each app is using. Go to Settings>Celluar and scroll down to see how much celluar data was used since the last time you reset the counter. Also, each app you have will show how much data was used.

e-mail doesn't typically use that much data, unless you're sending and receiving lots of photos and videos. Same with the Messages app. Facebook and Twitter can consume a lot of data if you're watching videos and checking out photos.

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Background refresh of apps (introduced in iOS 7) will also consume data periodically. Also I noticed that when I wake the phone from sleep mode, the LTE is on before it switches back to wifi. Maybe iOS is turning off wifi after certain amount of inactivity time. Like others have suggested check the cellular data usage in the settings.

 

Even if you're not actively using your phones, cellular data can be used. Push mail is one example (and even Fetch does that, when you read mail), but others include Location Services, Notifications, and background app refreshes. These can be constantly communicating to servers on the web. Many of these features can be very useful, but not every program that uses Location Services or sends Notifications is worth the data expended. Go through those settings, and turn on/off those features on an app-by-app basis.

There are all sorts of ways to fine-tune your cellular data use. You can turn it off altogether, adjust the settings mentioned in this thread, and even decide in some cases whether that app is allowed to use cellular data (such as for downloading apps, or music).

 

Even if your phone is on Wi-Fi when you put it down for long periods of time and it's "asleep" when background apps refresh they use cellular.
You gotta turn background refresh off.

 

Setting > cellular

See what used all the data

 

Even though I'm on an unlimited data plan I still turn off cellular data when I'm home.. To save battery life.

 

The data was used.
They all claim they didn't use much or any data but it could also be the other person that used up the 1GB of data and playing dumb. You didn't pull 1GB worth of email messages that's for sure.