> And where do you draw the line between merely an app or website and a digital "public square"? Certainly Facebook or Twitter seem like the latter by virtue of their size and reach, but what about Hacker News? Mastodon instances? The Something Awful forums?

I favor the framing "the internet as a whole is the public square" over framings such as "this social media company is or is not a public square". A particular social media company has no inherent social obligation to host a diverse range of opinions. Governments cannot tell social media companies to moderate in a certain way, but social media companies should be able to moderate however they want to based on just about any motivations.