Exactly, it's a 1st amendment issue because it's a monopoly (or mutual anti-competitive practices) issue. So you're entirely correct. If the big tech companies weren't engaging in anti-competitive practices, then the censoring would not be a big deal. But because there really is only one Facebook and little to no competition, and they're colluding in the same prejudice, then yes, it's a lack of freedom of speech.

I'm all for the "they're a private company, they should do what they want" argument, if they (Twitter and Facebook) weren't the only nationally agreed upon means of communication and socializing during a nationwide lock-down.