Actually, a city square does exactly that, amplify your voice to all those that pass. The difference is that there are many city squares to stand in, whereas there are only a handful of social media companies. The problem here is Facebook's monopoly power, not the medium of communication.
I’m just not sure this holds up to any kind of scrutiny. There are tens if not hundreds of thousands of forums online where people can speak.
It’s a problematic analogy anyway, Facebook is not a public space, if we’re going insist on applying a real world physical analogue, they would be much closer to a city’s incredibly popular nightclub with a stage, which of course would never be required to give extremists or provocateurs free-reign of their equipment or stage. But even this analogy has problems.
I think the real argument is whether or not a company is free to associate its brand with whatever it likes.
So this is different from walking past a demonstration or assembly in the city, it is more of an Inception movie-like warping of the entire city to bring everyone closer to these peoples voices.