In order to get user emails you must request users permission when authenticating, so, when you use the Login action in the Facebook object include "email" in the list of permissions you're requesting. You can find a very complete list of permissions for the Facebook Graph API here: https://developers.facebook.com/docs/fa ... ermissions.
Also, remember that, as mentioned in Facebook Graph API manuel, "All these permissions, except the default, public_profile, require that you have Client OAuth Login enabled for your app on the Facebook Login tab of your app dashboard."
Once you have the permission you can do a request to the Facebook Graph API via AJAX.
Hope this helps. Cheers!