Furthermore, it's not just about targeting but about silent targeting. I'm not the target audience for Fox but I can still, if I choose, see what they're saying

Furthermore, it's not just about targeting but about silent targeting. I'm not the target audience for Fox but I can still, if I choose, see what they're saying and thus know what sort of (mis)information other people are being exposed to, and possibly respond accordingly. But silently-targeted ads mean people can be shown lies all day long without anyone else ever knowing about it. You should be opposed to this even if you believe strongly in the marketplace of ideas, because one of the bedrock principles of the marketplace of ideas is that people can fight false claims by responding to them. But if they aren't aware those false claims even exist because Facebook is secretly only showing them to targeted subsets, that assumption is broken.