Your data (especially the fact that users clearly prefer it) tells me that they're clicking it out of curiosity, to see how they can log in with their email.
Unless by "clearly prefer it" you don't mean the initial button click, but the final login?
Unless by "clearly prefer it" you don't mean the initial button click, but the final login?
* Facebook * Google * Twitter * Email
We don't use the persona messaging, and I think people's expectation when they click the 'email' button is that it's going to just be a normal email flow. We don't call it Persona or Browser ID or use any of their assets or messaging, because we didn't think anyone would click on it if we did.
But yes, we see a small preference for a button labeled 'email' versus facebook, and a medium preference for either over twitter.