Facebook Connect offers other advantages to Persona that have nothing to do directly with login: you get a profile picture, a name, and a gender. You get this same information from other value-added login providers, such as Google. This dramatically cuts down on the complexity of a sign-in process for both the user and the website. You additionally have fairly rapid access to a social ecosystem (which I realize not everyone wants; I have personally never taken advantage of this on my site), and can start doing things like "friends of yours did this thing". This means that Facebook Connect is something a website might use even if they consider login to be a non-problem (and I think patio11's point is that login is largely a non-problem).

Even just for login, though: Persona doesn't solve this problem as well as Facebook Connect (from either the perspective of the site administrator or the user) as it assumes email addresses are canonical and unchanging, which is not true for normal users (who tend to change email addresses occasionally to get rid of spam or old contacts they no longer like; they also often get their email addresses only temporarily from schools, employers, and network providers). Please understand: I have had the same email address since the late 90s, and likely most people reading HN care about canonical identifiers, realize the security issues involved with losing their email accounts, and often have business reasons to not lose access to old contacts--we are not "normal people".

With Facebook Connect the user normally cares more about their account (people change Facebook accounts much less often than email addresses), and when their email address changes they can update it on Facebook without bothering the site they are using. (I run a site that uses only external login providers, either Facebook or Google, and the users who log in using Google often contact me saying they changed their email address and now can't log in to my site anymore as they had to make a new Google account. I frankly only offer Google because some people hate the very idea of Facebook.)

This undermines one of the major advantages of using third-party authentication providers. For site administrators, it means you still need some way to identify and authorize accounts separate from Persona (so as to know the user contacting you, who no longer had access to their old email address, is really the user they claim to be), and for the user it means they still have to find all the websites they have previously logged into in order to correct their email address, and they must do so manually and painfully (potentially talking to a customer support representative).

If all Persona did was "email verification" that might actually be kind of awesome, and then have that tied to an existing login system (maybe one involving a username and a password) provided by the site (or even by a third-party), but it ends up being sufficiently intrusive to the flow that it just doesn't feel right to use it and another login mechanism, which means that these killer "email changed, don't have access to old e-mail address" issues become entrenched. That also isn't how Persona was marketing itself, so no one seems to think to use it that way (leading to the "it doesn't seem to better solve a problem I really have" issue).

(I have written more about this subject in the past, with at least a small amount more detail on some of the other problems that can arise with relation to email accounts provided for temporary use, such as by the aforementioned schools, employers, and network providers; the short version, though, is that email addresses often get reassigned, which makes the entire idea of using an email address as the basis of what is supposed to be a widespread secure login mechanism insane. OpenID has tons of its own adoption issues, but at least it required providers to never reuse their identifiers between different people, and provided a simple way to allow that to be possible... I mean, even Yahoo and Hotmail expire and recycle accounts, so this is a serious problem.)

https://news.ycombinator.com/item?id=4233391

https://news.ycombinator.com/item?id=4605507