So you've got a mail stating somebody invited you to an invent via Facebook. Since you don't want to type that by hand, heres how to get the invitation into your calendar:

• click the link in that mail (or go to the event or go to the event via the webinterface of facebook)
• Right over 'Your Answer' there is 'Export'
• Click it, a popup opens
  • choose to send it to your email account (which should end up on your citadel somehow)
• shortly after, you will find a mail in your mailbox with the subject 'Calendar entry for some event'
• Switch your Iconbar to Roomlisting mode
• open “My Rooms”
• drag the mail from your Inbox to your Calendar room.

Done! The next time you open your calendar, this Event will show up.