This is me on Mastodon
Twitter - Assumed public. Everything with the exception of private messages is public and searchable
Whatsapp - Assumed private - Everything is only available to the users that are sent it.
Facebook - A mix - Messenger messages are private to reciepients, Posts on timeline are limited to those that they are shared with but can be public, Posts in groups available to anyone who joins the group (dependent on admistrative control)