I don't use Facebook. But there was once Hiroshima/Nagasaki discussion that I wanted to participate in, and you could only comment using Facebook. I created a dummy Facebook account just for that conversation and then abandoned it.
Since the blackmailer hit me too with a password that I don't remember, perhaps it was the password that I created for my throwaway Facebook account.
One way of knowing if the breech came from Facebook would be if lots and lots of Facebook users report receiving such blackmail emails.