A very good question, one that I've yet to find an answer for I'm afraid... I've been wanting to do this too, as I've got both FriendStream and the official Facebook app running since I couldn't get the Facebook app to properly sync contact avatars. This means that they're both putting entries in my calendar, which in turn means that I've got everyone's birthdays twice in the calendar.
I really don't know. Guess we'll have to wait for the Facebook app to be updates or something.