This question may be too general to lead to useful replies. Here are some things you might want to disclose, or at least think about:
What kind "event" is this?
Who is your audience?
Do you have a minimum/maximum number of participants in mind?
Where will it take place?
From how wide a region do you hope to draw participants?
When you say "social media" what, specifically do you mean?
Do you mean you want to promote your event via Facebook-LinkedIn-Twitter? Groups similar to those but not so big? Groups such as those you find on meetup.com? Forum sites such as this one? Your or your client's blog? Something else?
Are you planning to advertise on any of the above, or is a virtue of "social media" as you see it that it's "free"?
How well-established is your (or your client's) presence on the social media forms you plan to use?
Basically, social media promotion won't do you much good if you don't already have a following or some other way to demonstrate your authority. It's not realistic to believe you can start a blog and set up a FB business page today and have a full house at your event tomorrow--or even in 2 weeks.
If you or your client don't have much experience with social media promotion campaign, you do also need to be prepared to try (and track) many different possibilities. You also need to experiment with timing--I find that announcements and enticements placed on certain sites generate better interest with a longer lead time between notification and event, while others get better response with less.
If you build it they will come, but build is the operable word here.
I don’t know about other sites but I give here you six steps to promote your event on Facebook.
1. Add social share buttons to your event registration page
2. Add a blurb to your Facebook Page banner
3. Promote your registration page on your fan Page wall
4. Create a Facebook Event
5. Offer live micro events on your fan Page
6. Encourage attendees to engage and share
If you want in depth info for this your can visit marismith.com/six-ways-effectively-promote-events-on-facebook-case-study/
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.
Creating a Facebook Event should help. Or you could start a fan page altogether and invite people to like your site. Set up a compelling welcome tab in the fan page where people would get more info about the event you are promoting.
Make sure you find and advertise to your target market. The more people you reach, the better your results, but it is still better to reach people that will be interested and most likely to attend your event.
Regardless of the size of the event, set up a Facebook page or create a dedicated event sub-page under your Facebook page. It costs nothing and gives you access to the extended social networks of registrants and potential registrants. When people “like” your page, that action is shared with everyone in their network.