The founders didn't intend anything for "free speech" save for what they enshrined in the constitution. I'm not sure how that could be more clear. Facebook is not a public forum. Back in the founders time, the only public square was the actual public square. There were even fewer publishers with even fewer opportunities for people to have their ideas amplified and disseminated, and even back then they did not think that xyz printing press had any obligation to publish anything anyone wanted to.