Does leaving apps open on the iPhone use data?

Is there Internet apps? Like the Facebook app am twitter app?

It depends on the app. Most don't.

Don't think so, just when your actually using.

It uses RAM but not hard disk space

Program that are running, even apps, are in the form of data. They can't exist without data.
When they run, they need Memory(RAM) and a processor.
And to run all of that you need power. The battery.

I'm afraid so. It has been known to drain the battery. If you want to preserve the battery, close them (double click the home button, and press and hold an app until they jiggle.)

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