Do you mean it should be seen as a public space?

I agree there is an argument for regulating large for-profit social media sites as a public utilities... but I can't see how they are "a government" in and of themselves.

Not quite. I mean that it has rules of behaviour, it hires people to enforce those rules, it has strategic objectives and engages in world events, and is an economic force just above the middle of the IMF 2018 list, roughly equivalent to Slovenia (assuming GDP ~= revenue), or North Dakota if prefer to compare against a state of the USA.

Facebook is not a nation, despite all that, but not all government are national governments and I think it counts as a kind of government.

> I would argue that when a website get enough users, it should be seen as a government

That's absurd. It's a website not a government; nobody is forcing you to use facebook, if you dislike how they operate their website don't use it.

I’m British. Nobody’s forcing me to use the British government, as evidenced by the fact I left the country with no intention to return. (Some people have even given up their citizenship). Sure, there are things that makes it hard to do, but that’s true for leaving FB too.

That's an absurd comparison. It's a website not a country.