UX is one major problem with Persona. I built a small site that used Persona a while back (without offering any other login choices) and most user can't figure out how to sign up.

Facebook login as a separate brand works because user already knows and uses Facebook. This is completely opposite in Persona case. User don't already have Persona "account" and they don't understands why should they sign up for it.

IMO, Persona should aim to replace classic login instead of social login, e.g. by making Persona nearly invisible to the user.