Private companies are morally bound to uphold free speech norms, and in some cases, they're legally compelled to do so as well: see Marsh v. Alabama. It seems obvious to me that social media companies provide "virtual town squares" and that sooner or later they'll find themselves bound by Marsh- or Pruneyard-style requirements to uphold free speech. The alternative is to have a de jure but not a de facto right to free speech.