How would Facebook use 2.7GB without uploading video?

I have a coworker who asked me about his Galaxy S5. He showed me how Facebook has used 2.7GB of mobile data alone (5.71GB of Wi-Fi!). He's a light user who reads his news feed and uploads one or two pictures a week. His video streaming option is set to Wi-Fi only, and Facebook isn't syncing contacts or photos. He isn't rooted, and has everything stock; he hasn't even changed launchers. Not a tech user at all.

One thing he does that seems strange is, after he's done using Facebook, he logs out. I don't know how strange that is. I've never logged out. I secure my phone, but not any of my apps. This guy doesn't mind putting his password in every time. The only thing I can think of is that logging out clears the app's cache, but why would it?

I told him to use Chrome and showed him how to enable data compression, but I don't think that's going to help too much.

I checked my own Facebook usage (HTC One M8) and Facebook has used 0.07GB. So like, 70MB. That sounds more like it.

I'm shocked by the 2.7GB, but the 5.71GB is just nuts. Or when you add them together to get 9.4GB. Unless he's lying and he actually is recording video at the GS5's highest capacity (2K?) and uploading hours and hours of it... I just don't see it.

Just thought I'd throw it out there. It's a pretty niche case.