If *there is no longer any way to stay in touch with people if you leave Facebook* then that, alone, should be more than enough for anyone to realise what has been going on and establish alternatives. Same with X or anything else to be honest.
If someone online is really your friend/family/colleague, they will be happy to give you their phone number or email address if you ask. If they aren't happy to, then maybe they're just a rando after all.