How does Facebook, Twitter etc use MB on my iphone?

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In case you didn't know, MB stands for megabytes, and you use megabytes as data usage.

Every app you use, whether you're using Facebook, or even Apple Maps, it uses data when you aren't connected to Wifi.

You know you're using data when there is a circle going round and round next to the 3G symbol on the top black bar on the iPhone.

There is no way to decrease it, as you are using data anyway. In most cases, you'll never use more than you're assigned in your data plan, so it isn't something to worry about.

If you are REALLY worried about, look into an app called Onavo Extend. It greatly decreases your data usage, without having to do a thing. Download it in the App Store, and follow the on-screen instructions.

Hope I helped!