The Facebook new Send Dialog lets people to send content to specific friends. But the API @ https://developers.facebook.com/docs/reference/dialogs/send/ said the parameter \
I'm not sure how you're doing it or what you define as "works", but the below kind of works:
Go to the javascript test console here: https://developers.facebook.com/tools/console/ And type in the following, and hit run, it will bring up a send dialog with two names prepopulated. Unfortunately, when you hit send, it goes to an error page (still trying to figure that one out). But still, it does populate the dialog with two names :P