i think it isn't and that s why i think we need Persona, but without the email-account requirement. Plus mozilla canned the project too early and without ever really trying to push for it, as if they didnt want adoption.
I think such a system is needed, we need to teach people that they are in control of their identity and that it is independent of facebook. Kind of like how mobile apps do it with numbers.
Email is already the decentralised identity protocol.