In my opinion, this is one of the most interesting questions on here. Facebook's default method of identification is and always has been (since thefacebook.com times) an email address.

If they are launching their own email client, it will be silly for them to require you to provide them with another email address in order to use their site. How will they authenticate people? Or they won't and there will be a ton of spambots with facebook accounts. Either way, I think it's an interesting question and I'll be interested to see their solution.