@inventor aha! Then I misunderstood your objection.

I will say that while it's true, I think it's a bit pedantic. The OP wasn't suggesting, afaict, that "social.any.domain" clearly would be official, but that "social.known.domain" would be clearly official, and sensible - in that it's like the ".well-known" directory on a webserver.

Use the known domain, look for the social subdomain and voilà, there's the official social media accounts seems like a useful approach to me.