Susan Payton, of Egg Marketing & Public Relations, brings together on Mashable the ways in which social media can be used to promote real-world events like conferences, meetings, and parties. It's helpful to have a variety of options before you as you're planning an event, and Susan provides them, beginning with promoting an event by blogging about it.
Then you can post the event on Facebook, tweet about it, and give it a hasthag (#). While the event occurs, it can be streamed over Justin.tv or Ustream.tv. Afterwards, photos can be posted on Flickr or Twitpic. The latter can be posted even as the event unfolds to give people who aren't on hand a sense of what they're missing.
With all these promotional possibilities, Susan urges you to start planning an event long before it occurs. Involve employees and potential attendees in planning it with you. Not everyone has the same grasp of or preferences in social media, so make an event social beforehand.