There are many ways to promote you or your product through social media, but these are some of the most effective options available to you.

#1 - Facebook PPC Ads

Just like google adwords, facebook has a pay per click (ppc) advertising service that allows you to reach their 500 million plus users. Get Started Here

#2 - Social Media Contests or Promotions

Everybody likes free stuff, encourage facebook users to like your page by offering a contest or promotion. Check out Wild Fire and also Check out Votigo

#3 - Create & Interact with Facebook Groups

Start a Facebook Group, so that fans with similar interests can connect with each other. Get Started Here

#4 - Sponsored Blog Posts

Pay a blog to publish an article about your product or service, inexpensive and very effective. Check out Pay Per Post

#4 - Sponsored Celebrity Tweets

Did you know anyone can pay a high profile celebrity to tweet about their product? This is a very effective advertising method, but can be expensive. Check out Sponsored Tweets
In addition to this, you can use email marketing, video marketing and google adwords to directly promote your facebook or twitter page. Start now and start reaping in the benefits of a succesful social media campaign.