It is possible to send a private message via facebook- just not with the SDK. You can get the user's 'username' (ie. http://www.facebook.com/username) and you can send them an email via your app to 'username@facebook.com'.
Shapow!
Major bonus: attach a file and it will be included in the facebook message.
One caveat: the email will not reach the recipient's message inbox unless the sender's email address is affiliated with a valid facebook user account. I would imagine this is in place to thwart spam.
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