I don't think it's a free speech issue. It's just a scalability / UX issue arising from the problem that 100,000 people are trying to communicate in the same room, and 99% of those people have nothing new to add to the conversation. The self-moderation features on Reddit and HN are a step in the right direction. On Twitter and Facebook, there is no downvoting, which amplifies low quality content. This decision is tied to their revenue generation models, because they don't want to limit participation. Reddit and HN focus on improving signal to noise-ratio, which is more important metric, and I think even more can be done in this direction.