Addendum: April 13
The First Amendment normally is interpreted to the narrow legal concept of "publication" (a statement to at least one person who understands it). You submit an LTE, magazine article or book manuscript to a traditional print publisher, it is under no obligation to publish it and generally does so based on its own best interests as a business (will the book sell?) The publisher shares the First Amendment right with you. With hosted and social media platforms, publication happens mostly under the user control. Technologically (starting in the late 1990s) this had been unprecedented. The platform has the right to curate the content with algorithms, as part of its own First Amendment right. It has the right to remove or censor content or users whom it deems as unacceptable under its own political ideology, But it would probably not have the right to collude with other businesses to get them to do so and hinder competition. Judge Reid's comments starting on p. 51 probably help support that idea. YouTuberLaw (Lior Leser) is working on a complaint with the FTC over collusive behaviors that have allegedly been taken against conservative speakers, especially in late 2018 (as with Sargon of Akkad, Milo Yiannopoulos and others).