> A government which bans someone from speaking is preventing someone from talking to you.
I didn't say "prevents someone from speaking" - I specifically said "censors the press" because those are two very different cases (and because that's the exact language that GP used). A government could censor the press but not prevent private individuals from speaking to each other, so it wouldn't technically be preventing people from speaking - but it's pretty clear that it's still engaging in censorship (which was the point I was making to GP).
> If I am banned from sharing information such as classified intel
Classified material isn't covered under free speech. Free speech protects sentiment and expression. Classified information isn't sentiment, it's factual data, and it's protected for national security reasons, so as long as the material is classified for that reason, and not, say, to hide the embarrassment of a government official (which is illegal in the US at least), it's pretty clear that it's not free speech.
> A public square is one which is funded and governed by public means
I disagree. A "public square" is a forum for expression that appears to be open to all, styles itself as a public square, and has a substantial population in it. The CEO of Twitter, for instance, implied that Twitter is a public square[1].
> As a matter of prescription, why would we ever want Facebook to be the financially responsible party for public town squares?
Aha, that's a good question - we don't! Or, I don't - the problem is that it is a de-facto town square already, not that I want it to be (a regulated) one. Top of my wish-list is for it to not be that - for the DoJ to take it apart and/or force them to make it trivially easy to move your Facebook data somewhere else. (I suspect you share this sentiment) Unfortunately, because the DoJ appears to have lost its fangs over the past few decades, we're stuck with the Facebook behemoth - so, if we're stuck with it anyway, it's morally right for them to adhere to free-speech principles.
[1] https://www.cnbc.com/video/2018/09/05/jack-dorsey-twitter-us...