> already happened for Instagram

This is the case for business accounts but not quite for normal users just yet. You can create an Instagram account with a username and password and it will repeatedly nag you to connect to Facebook for a while but eventually give up.

I expect they (we) already are. I never formally linked my FB and IG accounts and don’t use FB itself anymore but IG always suggests me the people I left FB in part to get away from! It’s obvious that it has linked the accounts internally.

And if course they have my WhatsApp number from 2FA on the other sites.