Facebook Login Setting.

Login to Admin->Site Settings->Settings.

Here, you will be enable login with Facebook.

How to Setup Facebook Social Login and What is Facebook Social Login Callback URL?

Setup Facebook Social Login: https://docs.ultimatemember.com/article/20-social-login-facebook-app-setup

Callback URL:

if your domain is like that https://yourdomain.com/ then the callback URL is https://yourdomain.com/auth/facebook/callback

and

if your domain contents public like that https://yourdomain.com/public then the callback URL is

https://yourdomain.com/public/auth/facebook/callback