Private censorship is still censorship, and it can potentially become as big of a threat to social progress as speech repressed by the government. For example. lots of social issues have been avoided in mainstream media because of corporate/political incentives to stay quiet about the subject.
Not exactly. Sure, corporations could collude and tend to reduce the audience that one has access to. But those corporations, in all likelihood, would be unable to stop that individual from standing in a public square (on a proverbial soap box) and delivering their message directly to the unwitting public. So the individual still has an audience, but the audience may not be as big as the individual would like. Is that still censorship? No, I don't believe that it is. The person still has all the elements of free speech. The fact that the individual doesn't have as large an audience as they would like is really very much secondary.
On the flip side, if the corporations were obligated to help every individual reach as large of an audience as they would like, what do you think the practical consequences would be? To start with, we as fellow individuals would be drowning in a sea of literally billions of unwanted messages.
I know which side you are arguing for. But I am fairly confident that if you thought about it for more than 10 nanoseconds, you wouldn't actually want what you're arguing for.