It is remarkable to see a stream of tweets describing a particular event emerging in your Twitter feed, especially if more than one of the people you subscribe to are commenting on the same event. You really have the feeling of something exciting happening, even if it is just the monthly meeting of a local group.
Clearly it is in a company's interest to attract social media participants to their events. The only way I know to do this is to encourage your employees to participate in social media. That way when you have an event, employees can just post about it in Twitter, FriendFeed, FaceBook, or wherever.
Audience has to be built naturally, in the spirit of community. It is not as if you can just buy space the way you can with advertising. Social Media has to be carefully and respectfully cultivated.