The final act for Mozilla's Persona

I don't think Persona ever did much attribute exchange, but I'd disagree that this is a distraction from the main purpose. On the contrary, IMO it's one of the big reasons it failed.

If you are a website owner, implementing Facebook login tells you loads about your users. This information is really useful and valuable. What incentive do you have to implement a login system which not only doesn't do that, but has no _way_ of doing that? Privacy says that the design of a good system would have opt-in info sharing rather than automatic, which makes it a bit less attractive from a site owner perspective, but Persona didn't even have that.