As Upton Sinclair said, "It is difficult to get a man to understand something when his salary depends upon his not understanding it." When over a trillion dollars of value is on the line, history tells us that people will generally act poorly and come up with any plausible justification for why they're doing good.

This isn't to excuse Mark Zuckerberg, Sheryl Sandberg, or anybody else in a decision-making position. The first time your business model is widely seen to have played a determining role in a genocide [1], the ethical decision is unambiguous--it's a good time to step down and take your billions to lobby against that business model. There's no moral excuse for doing otherwise.

[1] https://time.com/5197039/un-facebook-myanmar-rohingya-violen...

Is that most ethical? I can think of several people where the world would be worse off if they owned a controlling interest in Facebook. Eg Ergodan.