Facebook often changes these steps so please also check:
You can sync your Facebook calendar with Google Calendar and DigiCal will automatically display your Facebook birthdays or upcoming events from the Google Calendar.
How do I export my friends' birthdays or upcoming events?
You can export your friends' birthdays from Facebook into other programs like Apple iCal, Microsoft Outlook and Google Calendar.1. Select Events from the left side of your home page
2. Scroll down until the end of the text on the right-most box. https://www.facebook.com/help/152652248136178
3. Choose to either export your friends' birthdays or upcoming events
6. Select the down arrow in Other Calendar and click on Add by URL
7. Paste the URL in the field: webcal://www.facebook.com/ical/.........
8. Select Add Calendar
9. Make sure you have also selected the Facebook birthday calendar or the Facebook upcoming events calendar in DigiCal on both the calendars to display screen and the calendars to sync screen: