I have little experience with Facebook but it looks like they allow you to import events from ICS files (which is what iCal exports): http://www.facebook.com/note.php?note_id=290953877582702
From iCal you would highlight the desired calendar and go to File > Export > Export and save the resulting ICS files on your computer desktop.
Follow the above FaceBook article to upload the ICS file to the FaceBook calendar.
Keep in mind this would be a one time export / import. Any changes you made after that point in iCal would have to be re-export out of iCal and then re-imported to FaceBook.
If you want an automated solution, make a Gmail account and then add it to iCal: http://support.google.com/calendar/bin/answer.py?hl=en&answer=99358
You can then link your Gmail account calendar to Facebook: http://www.insidefacebook.com/2009/02/15/how-to-sync-facebook-events-with-google -calendar/
Changes made in iCal would then automatically show up on FaceBook.
Hope that information helps.