Seeing how much data usage an app generates is an indicator of how popular certain apps are (apps that don't get used don't generate much data) and it reveals how people are using their devices. If they're streaming videos, for example, that will eat up a lot more data than if they're perusing photos.
Unsurprisingly, Facebook's app generates the most data usage on smart phones. But YouTube rules on the tablet. It's followed by Netflix.
And here are all the apps people use like crazy on their mobile devices.