You are conflating ideas concerning the Bill of Rights. Anyone can contract away their rights and people do it all the time when they are hired.
Twitter, Facebook, etc., are private companies that you contract with in their "terms of service" agreement. To use their service, you agree to follow their rules. The issue is that these social media outlets are monopolies, and because there are no other viable alternatives, asking the government to treat them the way that other telecommunications companies operate is what the Internet Bill of Rights is about. For example, your cellphone company cannot deny you service because they don't like the topics of your conversations.