They can use other identity providers.

Hey, can you move this account over to this identity provider, please? No, I can't prove I own the account, but… Hello? Hello?

Yes, it's a footgun. Don't even start using FB as an identity provider.

What do you propose instead? And before you answer, consider that many systems only let you sign in with Facebook or Google.

Can you name some of them? All apps/services I've used always had some way of register account with email. Granted, this option is sometimes "de-prioritized" (displayed less prominently).

Advent of Code only permits GitHub, Google, Twitter and Reddit, despite using OAuth.

Well I think Google is a better choice than FB, but I can't think of any services I use where one or both of those are the only options and email signup isn't.

The only one that comes to mind (and that annoyed me the other day) is ProductHunt, which has 4 options for signup, but not email.

Please tell me you understand the difficulty of such an action if your attached identity to a site is inaccessible to you.