And Facebook connect already provides that.
So how does it solve a problem these sites actually have? Some small percentage of end users have this problem (the ones demanding a solution to privacy problems), but the rest don't think it's a problem, apparently.
(Surveys up the wazoo that show that people care about whatever mean nothing when people don't actually change their behavior)
That's not apparent at all. As far as I know, lots of people never even heard of Persona. I thought it was still beta. It's just in the works, so it makes sense not everyone jumped on board. There wasn't tons of marketing even.
A. Not everyone has an FB account. B. Those that do don't always trust that logging in with FB won't mean that the website they're logging in to will spam their wall.