It's similar to having the right to assemble peacefully in pseudo-public spaces like malls. But I would say the argument for freedom of speech on someone else's website is weaker than that; it's easy to just go to another website and say something there instead. Well, at least it's easy today. If Facebook succeeds at establishing a Facebook-only internet in parts of the world like India and Africa, then you might have a real reason to enforce that freedom, since those users wouldn't have any other choice.