What gives you the right to leverage their private property as your soapbox? Because people on the sidewalk won't listen, and that hurts your feelings? They have a business, if you are using your speech in any way to obstruct the conduct of their thoroughfare then they can have you ejected. The cops will not listen to your tirade against multinational burger tyrants, they'll drop you off at the drunk tank. Your speech will never be unconditionally protected, not online or in real life.
It's not McDonald's either, a singular location does not serve billions. The difference between a physical and virtual location is one of scale. As for your other paragraph, the point is that these corporations have gotten so large and people depend on them so much that being banned on them is essentially akin to being exiled from the ability to have free speech in modern society, whatever restrictions you want to reasonably put on them. What is the alternative you want people to use if, like Nepal, the government bans social media platforms, that I have not already addressed?
With your linking of that xkcd, it's clear you're misunderstanding my point about legal vs natural rights, as I stated initially.
But it's not at such a scale though. It does not have one location with billions of members.