I think your assumptions are incorrect: individually, the people who work at facebook aren't stupid people, but indeed, the company Facebook itself, and the decisions it makes, can be described to be the actions of a stupid company.
I think it is less stupidity than the monopolists' disease. It is a combination of arrogance and paranoia that you can see appear repeatedly in monopolies.
I mean, yes, ultimately counterproductive to monopoly-preservation. But it isn't quite stupidity.
