I wouldn't say it's unnecessary. Owning your identity is the worst problem in modern social networking. You have one identity. You. The idea you need to keep registering, providing emails, phones to everyone, picking usernames (often different) which can be impersonated, in order to create account which then can be banned at whim by private company moderators and so on... this is not the way.

I don't know if domains are the way either, but at least they're a name reservation system that's established and works. At the current price of a domain, it comes down to $1 a month, which is a great value if you can use it to identify yourself with, authenticate, authorize and own your own digital identity.

It can also be cheaper, because if this is a mass service for every person out there, scale will reduce some of the cost.