A public square is one which is funded and governed by public means, and the source of funding is one of the core mechanisms for ensuring that the public institution answers to public money.
Facebook is a private property, much like HN and the BMJ. That means people like dang, who also engage in banning, are not paid for by the government. Surely one's concept of public square does not entail private parties footing the bill for fancy engineers and metaphorical dang.
As a matter of prescription, why would we ever want Facebook to be the financially responsible party for public town squares? Why do we not have the government fund an actually public town square? Or ought we be using tax money to fund Facebook itself?
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...
And public funding is a critical component to public governance. Facebook is accountable to private money. There is a reason why people don't like the idea of politics being funded by private money.
To change that, we might talk about using taxpayer money to fund Facebook, and rather than mere rules for companies, we might talk about public ownership and governance of Facebook and not merely breaking businesses apart. Breaking Facebook apart is about dealing with free market concerns. Nationalizing Facebook is about recognizing that Facebook ought be a public institution.
Under the political theory underpinning the origins of the US, private institutions have different moral obligations from governments because governments have the monopoly on violence, not simply because governments are big and have lots of "users." Facebook can't force anyone to use Facebook, and their competition is always one click away.
Facebook isn't a "public square" because it has no local geographic monopoly on space; it's the Internet, we have our choice of internet fora. This differs from the physical world, where one can't easily just go to the next public square over if the use of the nearby one is curtailed (hence the need for legal theory enacting the concept of "de-facto public square"). There's a problem if Facebook is the only online forum. But the solution to that is anti-trust law, not the government dictating what Facebook may not refrain from rebroadcasting via its network. Trying to solve the legitimate concerns of discourse manipulation via modification to First Amendment protections opens a Pandora's Box that is almost certainly best left closed (for starters: given the customer behavior we currently see, one could infer that people use Facebook because they like the signal-tuning. Stepping on the neck of Facebook's ability to signal-tune in a heavy-handed way risks driving users to alternative services that don't operate out of the US and therefore aren't beholden to any concept of First Amendment protections, for good or ill).
"refraining from giving some sources a megaphone" is exactly equivalent to "censorship". There's no difference.
Your argument confuses the prescriptive and the descriptive.
Descriptively, yes, Facebook is under no legal obligation to treat speech on its platform equally.
Prescriptively, Facebook is morally obligated to either assume full responsibility for user content (both editorialization and moderation), or no responsibility (passing through legal challenges directly to the user).
Additionally, as a de-facto public square, the principles of free speech still apply - a platform with more users than almost any single nation on Earth has citizens is morally bound to allow free speech.