Yes, a bunch of times. It's on the advent of code website for example, which is pretty popular.

Even as someone who likes unique logins with email and usernames, I just double checked my Github account and I've used that to log in to 6 sites. As I finally deactivated my Twitter account last week and didn't check... I'm 9)% I didn't use "Login with Twitter", but I can't say for sure. If some site would only provide Login via Google or Twitter I may have. Probably nothing important, and I will find out sooner or later if I need to recover.

Yes, there are a lot of Japanese sites that use it.

My gawker account used it.

I guarantee there are at least thousands of people who rely on this. That's a lot of people's lives.

Yes. It was a DIY repair website and a few months later they gave it up. Can't remember the name.

ifixit had sso via yahoo et al as well and one day it just broke.