There shouldn't be one platform. Openness and interoperability are key.

Network effects say it would be at least an oligopoly, just as it is now. Unless the whole system would be rebuilt to explicitly disallow this and counter such effects, but who would build such a system and why would they invest in it, knowing in advance they wouldn't profit from it or even able to control it? It would surely face a huge pressure from all the "we can't allow bad people to speak" crowd, and this crowd owns the government, the academia, the banks and the internet infrastructure now. So what exactly the plan for it to happen?

I'm afraid this will be band for privacy - too many hands in the jar.