For most people, it goes like this: click on the Facebook login button, done. OR, click on personaID, have a new window open, enter email, enter password, go through setup, get back to site. This will be hard for most web users to adopt.
For other people it goes 'click the Facebook login button' - What? This POS uses Facebook? What access does it get to my Facebook account? Is it one of those weird things that posts all over my Facebook wall? [cancel]
Generally I've noticed people are fine with FB login as long as you add "Don't worry we won't spam your friends without your permission" or "We won't spam your feed, we just want to know it's you." or something like that
Yeah but once the registration hurdle is jumped, it's click, password, go. Some of us do prefer passwords and multiple identities in this age of homogenized identity.
That's not entirely fair. If you're not logged into Facebook, you still have to log in there as well. As huragok said, once you're past set up, it is just click, password, and go as far as I can tell.
However, I'm not arguing against that a lot of people probably _are_ already logged into Facebook.