I have a hard time following this reasoning. I would definitely consider Facebook partly responsible for this genocide if they had deliberately made an editoria

I have a hard time following this reasoning. I would definitely consider Facebook partly responsible for this genocide if they had deliberately made an editorial decision to spread the genocidal rhetoric. But they didn't. The situation was as you put it mostly unmonitored. In other words, the spread of genocidal content was organic. This is the equivalent of blaming the phone company or the ISP for what the users do with the communication channels.