AKAIK it is still possible with most popular email servers to send and recive using only IP addresses (e.g. by placing the address in brackets).
DNS is really only optional for email, at least functionally. But I think a lot of people assume there can be no email without domain names.
So why not regulate facebook and twitter instead of make another service that will essentially turn into DNS, which we already have?
Facebook and Twitter were funded under the assumption that they would be unregulated monopolies, so regulating them amounts to stealing from their investors.
Instead of another service that will essentially turn into DNS, I would propose just using DNS for naming. So if you're Tyler Durden you register tyler.durden.name and put an A record pointing to your DiSo server. Now you can switch providers while retaining your identity by just changing the A record.
The difference between DNS and Facebook/Twitter is that DNS is regulated so that bad guys have to go through some kind of due process to shut down your domain. It's not perfect, but it's better than what else we've got.