Once a company is classified as a public utility (I believe Google is) it should be forced to host all legal content. You tell me what's illegal and I can safely tell you it can't be hosted by Google.
Moreover, you're saying that spam should be forced to be hosted by these companies, just like our snail-mail protects. Even if it takes up Exabytes of information.
Should people who have their content removed be able to sue these companies for removing it?
Just with Twitter, we know that content is regulated by region. Setting your location to Germany will prohibit seeing certain content from the U.S. Many more such regional cases.
The spam example would be a problem, but it's more of an annoyance to solve than a basic human rights case. You simply cannot have a democracy where segments of the population are barred from interacting with public officials online. Especially when public business (advertising, fundraising, making political arguments) is now a core of online communications.
And effectively this would turn these sites into platforms that are so filled with trash they will be unusable. And the chaotic part of me would love to see that happen. But it means basically the end of these companies to function.
Realistically, I think we should keep to the standard we've had in the past: we can't compel companies to host speech they disagree with, and we should take strong measures to limit their anti-competitive behavior and break them up into competing companies if necessary (like we did with telecom)
I would say though, Twitters model from around 2012 was extremely open compared with today (remember the Arab Spring?) and in no way was it an unusable, trash/spam laden platform.
How? Create a cause of action whereby if a tech company removes someone's content, that person can go to court and ask that a judge determine whether that content removal is some kind of anti-spam operation or viewpoint censorship. You don't let the company have the final say.
No, those companies don't have first amendment rights. Commercial speech has always been more limited than personal speech. If the law worked like you claim, common carrier laws for railroads would be unconstitutional because they'd violate railroad company freedom of association. These laws are, in fact, constitutional, and so will be the laws that stop big tech censorship.
Besides: corporations? Rights? Total bullshit. This country ought to be run for the benefit of its citizens, not abstract entities like big tech companies. A corporation is an artificial construct that can exist only because society --- made up of humans --- determines that the corporation is in society's best interest. When a corporation is no longer in the best interest of the humans that make up the world, screw the corporation.