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.
(yeah, I can create a dummy account for that purpose alone, but I rather not bother)
[0]: https://developers.facebook.com/docs/graph-api/common-scenar...
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.
Also, consider adding an import feature from gmail, vCard, etc
Finally, why include male/female binary as the only required field besides name. Seems a little odd