don't spread yourself too thin. focus on the major ones:
1. facebook - have a fan page. join facebook groups and fan pages related to your events. interact with them and post your event linking to your fan page.
2. blogging and twitter - at least 2 weeks before the event, post on your blog and spread this to twitter and facebook. what should you post on your blog? discuss the benefits participants will get from attending the event. what will be covered in the event. who will be the main "speakers". discuss the topics in detail. you will never run out of blog posts.
3. linkedin - circulate the pre-event materials in linkedin. whatever you post, you also broadcast in linkedin.