I suppose it might be worth recognizing that a social media system with access to your private messages both gives them a lot of power, and confuses users. Privacy on social media isn't really trustworthy. If you want to whisper, do it via email, where at least it isn't on Facebook's servers.
I don't say anything on Facebook that I wouldn't want on the front page of the New York Times. I don't trust either them or other users to keep it private. (I don't even like putting sensitive things in email, if I can do it in person instead.)
There's no reason why a social media platform can't implement its private messaging in a way that is functionally equivalent to email in terms of privacy (but integrated with that platform).
I don't say anything on Facebook that I wouldn't want on the front page of the New York Times. I don't trust either them or other users to keep it private. (I don't even like putting sensitive things in email, if I can do it in person instead.)