Not. At least for the time being ...
If you look at the official documentation: https://developers.facebook.com/docs/reference/dialogs/send/ , you will see that this type of dialog does not expect a callback function as a parameter, and since there is no callback function, then there is no way for you to know who and how many recipients, and even if the dialogue is closed / canceled / sent.
What you can do, although it seems very complicated and good, the headache is to use the outgoing connection of the User object in the api chart ( http://developers.facebook.com/docs/reference/api/user/ ). I have never used this before, but I assume that you can (if you ask for read_mailbox permission) check the user's messages before and after the dialog and see what has changed.
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