I can see a website offering only Persona/BrowserId, as it doesn't force you to create an account with a "commercial" website. If you go with Facebook for login, you have to offer an alternative because if a user doesn't have a fb account, it has way more implications to open a fb account than to create a browserid account. I just hope Mozilla don't overgrow it into something more than a simple auth system (like building a social network around it!).
