I haven't worked for a company as large as Meta but we encounter these issues on smaller projects too: should the unique identifier for an account be their emai

I haven't worked for a company as large as Meta but we encounter these issues on smaller projects too: should the unique identifier for an account be their email? Or a UUID? If your business only uses SSO, should you use the [Google/Microsoft/whatever] ID as the identifier for that user on your database? I personally wouldn't think that Facebook engineers would have had their hand forced on something like this.