Bikram

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.