Email isn’t just used to authenticate, lots of critical processes including account recovery rely on discourse being able to send email to users.
I’m not aware of any supportable configurations where email addresses aren’t provided.
Procourse did work on a plugin to substitute email with SMS, but this has no ties to Facebook:
https://github.com/procourse/discourse-sms-authentication
You would need to check to see if a compatible SMS carrier is supported and test to see if the plugin still works.