[0]: https://developers.facebook.com/docs/graph-api/common-scenar...
(yeah, I can create a dummy account for that purpose alone, but I rather not bother)
I got added by someone else to a Facebook group that eventually went on a troll raid. I rarely go on Facebook and didn't even participate in any troll raids. Just from being in the group, my account was disabled. I have been locked out of my apps for three weeks and Facebook hasn't answered any appeal I sent them.
It's nice that I don't have to create yet-another-password for a new app as well. I use a password manager so it's not a big deal for me, but for other users it could be so they may just use a shared password and having FB login would be a way to have them not do that.
I have mainly python experience with python-social-auth, but I see PHP has a few pretty good social auth libraries, to name one, I came across HybridAuth[1].
I get that you might like it, that's fine, but it's not serving any good purpose here. Google login would be more widespread if you must have a 3rd party button. Yes, Google track as well and yes, that's also going to put people off, but if you feel the need to have one at all, get a more useful one. Or better yet, remove them all, have local user registration and put a hook in place for developers to plugin other buttons using different contrib repos. Then you can have both without polluting the core project with 3rd party deps.