This post will show you how to subscribe to your facebook friends birthdays using ical on your mac

1) login to your facebook account
2) goto events (in the left hand bar)
3) click on the maginifying glass in the top right
4) pick ‘birthdays’
5) click on the maginifying glass in the top right again
6) pick ‘export birthdays’
7) click on the url that facebook presents to you
8) pick your options when ical asks you to subscribe

you can get the birthdays calendar to update like any other calendar in ical, but it is only a one way sync – from facebook to ical.
changes you make in ical will not update facebook.