Does phone waste MB just for having mobile data on?

[Lucas]

If I have mobile data on but I ain't using any internet stuff, will it still waste MB? Also, if I have both mobile data and Wifi connected at the same time, and start using Facebook or whatever, will it use Wifi or 3g?

[Tom Hansen]

It can. If you constantly get updates every minute, then yeah. It does not waste much but it can waste at most 20 mb in a single day by just getting updates while your not on your phone. All of it adds up. i'm pretty sure that you can fix some apps to "give updates on wi-fi only" if you check the app settings. I at most waste 6mb in 8 hours if I have my phone in my pocket all day with 4g lte on.

[ScσττRΛSC³]

Technically. The phone would likely be doing things in the background like checking for emails, notifications from other apps, etc.

As far as having both on & which it would use it depends on if you are in a location where wifi is available to you. If so, then it would use wifi. Otherwise, 3G.