Arguendo, assume Facebook lobbied for it. If only one market player doesn't follow an industry standard on public interest (consensus which prevents industry regulation), of course other players should lobby to get the player out.
But it turns out the posts calling for Muslims to be killed were made by the military's propaganda wing and the killings were perpetrated by the military's armed wing. So the causal chain actually flows through the military, and Facebook was merely a place where you could observe it happen without being there in person.
Blaming Facebook for the genocide seems to be an instance of activists focusing on something they can see and try to influence (Facebook moderation policy) over something they cannot (guys with guns shooting people).