Unfortunately the e-mail information that you are trying to get would be a part of the profile of the user, and that is not a field that is set or returned within the profile. Since the profile system runs by the user GUID, which is not associated with a user's e-mail, the e-mail will not be returned. So, I don't know of a method through our social APIs but if there is a way that I discover I'll let you know.
- Jon