I had this question too (especially since iCal has that tantalizing "Accounts" tab in the prefs that doesn't support Facebook). However, Facebook gives you a webcal link that you can put in iCal. Instructions here:
Make sure when you add to specifcy "On My Mac" as the calendar location rather than "iCloud". Otherwise, using iOS6 and iCloud, you might see duplicate events.
Yeah, that is the work around that I figured out as well...
I would like to see TRUE Facebook integration in iCal meaning two-way syncing with facebook events --> create event and invite facebook friends IN iCal, and it shows up on your Facebook thru the web and vic versa.
Apple Software Dev team, we are patiently waiting...