Whether or not a post is available to "the public" (signed in or not) is completely the choice of the author, not some mysterious committee at Facebook.
On loading a page, I get a modal popup obscuring all content insisting I "register" or "login", with a "not now" option. Then when you click "not now", the bottom third of the screen is obscured by a fixed overlay.
It is loosely describable as readable, but in practice, it's a fight to read it. The element has randomly generated id, so can't easily be blocked with ublock or similar.