* Google, Facebook, Twitter, ... have become so well-known brands comparing to the new Persona. Normal users could quickly get a sense of what "Sign in with Google/Facebook/Twitter/..." means, but not "Sign in with Persona" means.
* Persona is not a social network while other mentioned brands are or provide a sense of social network. A user might perceive "signing in with <your-favourite-social-network-provider>" as an act of making the site a part of the network; while with Persona it's totally different.
So if for instance, the user enters his email and is using a persona identity provider, e.g Yahoo It could just give a message 'Sweet, why don't you login with Yahoo " or create an account.
If the user has a Persona account already, once the email is put, it could say "Perfect! You are logged in"
If the user is using persona for the first time and does not user an identity provider, it could just bring a persona form.
Of course, for each instance, you could have a tiny "powered by persona" somewhere. With a bit of thinking it can be refined.
I do not see any reason why a user will want to start thinking about what persona is. They will just use an alternative (Facebook). What persona should be aiming for should be to become "login with email" and not another 3-in-one brand called persona.
Think about it this way: suppose you create a persona.org account at site X, then visit site Y which also uses Persona for login. It would look like site Y recognized you, but how? Seems like an incoherent user experience.
Does this help at all?
Firebase offers a login service which includes Persona alongside Facebook, Github, Twitter as login options. They've got a demo here: http://firebase.github.io/firebase-simple-login/