I suspect the reason why people have stopped posting is because of increasing content from other sources. It's definitely my reason.

It's a negative feedback loop.

Yep, one of the main reasons I stopped using FB many many years ago was that they made it so hard to view the posts I actually cared about, and gave the user almost no control. It was clear at least a decade ago that FB didn't care about users and just saw them as chattel to squeeze for money.