You've experienced this: you RSVP for an event on FaceBook and it presents you with an option to "Export" the event. Of course, exporting means that you can download an iCal (.vcs) file and import it into iCal, Entourage or Outlook (if you're lucky).
But what if you're using Google Calendar? Simple: email it to yourself.
If you use gMail with your Google Calendar (as I do):
1) create a new message, address it to yourself, attach the iCal file and hit send.
2) When the message arrives, you'll notice that it contains a small calendar icon indicating that it is a meeting request.
3) After opening the message, click "Add to Calendar".
4) You'll be prompted to add to the event to your Google Calendar. Assuming you don't want to make any changes, click "Save Changes"
5) Google Calendar will confirm that the event has been added to your calendar.
6) Check your calendar to be sure that it's been added.
If you're using FireFox, you can get a Greasemonkey script that will automagically import your Facebook events into Google Calendar. Thanks to Matt Stevens (@GadgetVirtuoso) for the tip!