Add Facebook as an identity provider for External ID
You can add Facebook to your self-service sign-up user flows so that users can sign in to your applications using their own Facebook accounts. To allow users to sign in using Facebook, you first need to enable self-service sign-up for your tenant. After you add Facebook as an identity provider, set up a user flow for the application and select Facebook as one of the sign-in options.
After you add Facebook as one of your application's sign-in options, on the Sign in page, a user can enter the email they use to sign in to Facebook, or they can select Sign-in options and choose Sign in with Facebook. In either case, they're redirected to the Facebook sign in page for authentication.

Reference:
https://learn.microsoft.com/en-us/entra/external-id/facebook-federation