Yeah, which is why using their oauth is a great way to stick it to them! Obviously don't only use it, but giving their users a one-click way to leave their platform is hilarious!
I understand that you are not fans of Facebook. But couldn't this be a good opportunity to show Facebook users that there is an alternative out there? I'm just not sure that restricting users who primarily use Facebook oauth does any good, to me it feels like a wasted opportunity!
I think the only reason to include FB OAuth is if it allowed such users to integrate the service with FB Events (which, AFAIK, isn't possible with their current API).
I'd say it's a given that FB users also use Google, and would be just as likely to use either service's OAuth.
I think that's a safe assumption. Anecdotally though, I prefer to stick with one service and if I was offered options that didn't include my primary choice I might be reluctant to sign up. I just don't think that intentionally walling out a group of potential users is a great idea if you're trying to expand your audience.
I tried to quickly find some numbers on distribution between them. Although I didn't find anything concrete, it seems to be Facebook > Google > LinkedIn. I would actually love to know more about this. Maybe someone with more experience has some more data?
