The definitive answer to this is from Facebook themselves. In post today at https://developers.facebook.com/bugs/335452696581712 a Facebook dev says
The ability to pass in an e-mail address into the "user" search type was
removed on July 10, 2013. This search type only returns results that match
a user's name (including alternate name).
So, alas, the simple answer is you can no longer search for users by their email address. This sucks, but that’s Facebook’s new rules.