Free speech doesn't have a single meaning. Freedom is relative. There is no right to absolute free speech anywhere. For example, you can't yell "fire" in a crowded theater. On social media owned by private corporations, "fire" isn't the only thing you aren't allowed to yell. Subverting democracy, inciting violence, propaganda from foreign governments pretending to be grassroots movement inside the US, etc., are all banned, and yet I would consider Twitter a free platform, albeit with sensible limits. But you're right, that's not absolute freedom.
Fire in a theatre came up in the context of squashing a protestor against the draft, from a judge who thought eugenics may have something going for it.
