I feel like people over-index on the idea of "fiduciary responsibility to shareholders." I don't think most decisions a CEO makes are really done with shareholders specifically in mind. They do what is best for the company --for its health, growth, and sometimes survival-- and shareholders are just there for the ride. Certainly, there are some executive decisions where shareholders are the primary factors, but by and large I think they're an afterthought at best.
He over-hired, he lost faith in the direction of a big chunk of Meta's initiatives, and he now wants to pare back.