If you’re logged in to Instagram or Facebook, and you copy a link to paste into a MN thread (or indeed anywhere), anyone clicking that link can see your profile. It pops up saying “Joe Bloggs sent you a link. Add them as a friend?” and your profile picture is accompanied.
Ive seen it loads and didnt wanna post on individual threads telling people as it feels like an invitation to check out who that person really is. Posting in AIBU for traffic - it’s shocking IMO, but thought I’d warn people who didn’t realise. Not anything MN can do about it, it’s a Meta issue.