FB and Google are huge companies and control majority of the information people get today. We should want them to be moral arbiters. Otherwise what else can we do? Most of us are so gullible.
A completely hands-off approach is also corrosive and destructive. They're stuck trying to thread the needle, to balance civic responsibility while avoiding being an overbearing gatekeeper. I don't think it's possible to pull it off in such a way that they are not overstepping bounds in one direction or another, but as long as the pros outweigh the cons of whatever approach they take, it'd still be infinitely better than doing nothing.