I think the counterarguments on Persona are really valuable. On Facebook, I have 2 major concerns. 1. It's not guaranteed that a user has a Facebook account.

I think the counterarguments on Persona are really valuable. On Facebook, I have 2 major concerns. 1. It's not guaranteed that a user has a Facebook account. I don't mean in general, but similarly as people change their email address when it's full of spam, people shut down Facebook in response to events in their life, like a breakup, to maybe recreate a new one later. 2. I'm absolutely not convinced about the fact that people update their email on Facebook. In our experience with Facebook login, in fact, the email (even when certified by Facebook) is never reliable and it's usually that very old email you had when you signed up to Facebook ages ago. IMHO this is because Facebook is so complete in terms of communication, that you don't actually need an email address for using it.