She's the de facto CEO. She's placed there by investors to manage Zuck and stop him doing anything that might jeopardise all that IPO capital - and after her time at Google, she proved she knew how to manage ASD idealists. I'm rather glad this has happened, someone who built the two biggest douchebag business models the world has ever seen is not fit for public office.
The scrutiny on her as COO is disproportionate here.
Can you even name an equivalent person at Google (now) or Microsoft? Or a presently troubled company - Tesla, maybe? I'm doubtful, and I'm willing to say there's probably a sexist element to this inappropriate scrutiny.