Although a couple of years old, this post is a great resource for those looking into social media marketing.  As with any other venture, the first thing you need is a PLAN–don't just sign up for Twitter, Facebook, Blogger, etc and expect your sales to take off.  Consider the resources that will work best for your product, develop a strategy, and then start building out your profile. 

A couple of the hottest resources out there are, of course, Facebook and Twitter.  However, don't overlook the potential of a blog focused on providing value to your customer base, and consider niche areas or "microcommunities" as well.  If you sell coffee-related products, you are much more likely to get targeted, interested traffic through participating in forums on coffee-enthusiast sites.  Just remember–careful planning is essential–if you start blindly spamming the forum, you will not only turn people off but likely get banned.

In my opinion, social media marketing is more about the "soft sell"–offering value, creating a good reputation–rather than the brazen "click here to buy" approach.  If you make your business attractive, customers will search you out.