Yes, these are problems you come across in a nanny state. Which is why in the US, most all speech is protected, even hate speech. And for the government to impose laws or rules to force a private company to regulate that speech runs counter to that. So I'm not really sure why this is a conversation in the first place.

there is a large set of rules that define what can't be said or shown on radio or TV

Only on TV that is publicly broadcast over the public airwaves. Anything private like cable or satellite can say anything they want.

> Basically, Facebook should be taxed for the externalities they create like fake news

All economic activity produces externalities, positive and negative. That doesn't mean we should increase taxes and subsidies on everything.

I think parent's point is that "fake news" is a negative externality of the same kind as pollution.

Which it isn't. Pollution is a well-defined term; you can identify particular offending substances or disposal methods by their impact on humans and their environment. There are precise things that can be taxed to discourage their use. "Fake news", on the other hand, is a pretty uniform spectrum, from the most blatant falsehoods to the stuff CNN and NYT write. It's hard to even agree on where to draw the line (personally, I'd draw the line at "journalism should begin and end at stating verifiable facts + opinions on those facts clearly marked as opinion", but that's an extreme/minority position), and there's no way to start quantifying impact.