they have to be connected to a contact in order for their birthday to show up on your calendar.
IMHO they should put the birthday under the contacts info, but thats another story.
Facebook only connects contacts if it can determine that they should be connected. I believe it does this by matching the phone number in your phone with the phone number in their profile. If they don't have a phone number in their profile you are aout of luck. It may also look at e-mail addresses but that is probably it.
Facebook actually checks the names and email addresses in your addressbook to your friends list and adds them that way. So, if the name you have listed in your phonebook isn't how its listed on Facebook, you have to manually link them together
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