Hey Raymond! 
I’ve spend quite a bit of time in the GoTrue and GoTrue-js libraries over the last couple of weeks. Between our prior thread and another, hopefully I can point to the code behind Identity answers!
It’s a good question though and it was a fun rabbit hole to jump down for an hour 
The tl;dr is that while the Facebook provider was implemented in the GoTrue / back end service some years ago, we don’t actually know if it works or not and there’s no way to activate it as far as I can tell. Even trying to tap directly in to Netlify’s core API and update the Identity service settings to set facebook: true - I don’t think the Netlify core API is even allowing that value to propagate down to the GoTrue service. There’s a solid chance the GoTrue provider implementation is out of date though, and who knows if Netlify still has valid standing with Facebook’s API settings as an app that can be integrated.
So… you’re all right? There’s a lot of variables on all ends of the workflow as far as I can tell. The GoTrue service does actually respond with a URL to Facebook that would indicate a token request but it doesn’t contain app credentials. I’m not going to test it but there may be a chance that if you sign up on Facebook as your own app and inject that key into the redirect response from GoTrue, maybe that’ll work? Shooting in the dark here.
Anywho… Facebook isn’t implemented in the Netlify Core API, Netlify Dashboard, or Netlify-Identity-Widget, which most folks use. Gotrue-js supports it from a generic standpoint but it’s just a pass-through to GoTrue anyway 
–
Jon