It seems that Meta assigns a unique hostname to each shared conversation. For example:

> fiscal-review-b4a5ff.cf.metaaiusercontent.com

So when someone opens the link, the DNS resolution exposes the unique hostname. The original bug reporter found dozens of these URLs in passive DNS logs.

That is remarkably amateur. I'm not a software engineer by trade nor have worked at a billion dollar company, but knew that to be a bad design within seconds.