Organize an Event on Facebook

"Once your event is up and running, you have two options: let it stagnate and hope people show up, or get actively involved in promotion and management. For anything but a small, private get-together with friends, the latter is usually a better option.

If your event is open and allows guests to invite others, you should encourage that behavior if you want word of the event to spread virally. Send messages to those who have attended urging them to invite friends and help spread the word (you can send messages to invitees by clicking on the “Message Guests” link below the event photo on the event’s page). You can also send messages specifically to those who have not yet replied to the invitation or to those who have indicated that they may attend urging them to come to the event. For open events, guests can click the “Share” button, to share a link to the event page on their mini-feed or send a message about the event to friends."

http://mashable.com/2009/10/14/facebook-events-guide/