How do they show it? We don't know how many % of the iOS user base uses Twitter or any observable channel to praise iOS 7 or complain about it.

Plus it's impossible to downgrade, so any iOS 7 user in a statistic might be an unhappy one; or the other way around, maybe I would love iOS 7 on my iPad 3rd gen, but I won't irreversibly upgrade for now.

That's not a study of actual users, but the application of random metrics plus a bit of personal opinion (in the user friction section). It's as good or bad as the OP.

This is what I'd love to see, regardless of the results: http://www.nngroup.com/articles/windows-8-disappointing-usab...

As far as I know, the closest thing that's publicly available are customer satisfaction surveys.