To answer that we need to clarify what we mean by social marketing in this instance.
Social marketing is a concept whereby organizations promote an idea for the benefit and social good of society as its primary aim and not to make a financial return – like a health campaign for example.
Social marketing is also a more modern concept where a business promotes their products and services on social networking and video- sharing sites, such as Bebo, MySpace, Facebook, and YouTube, for profit.  This is now more commonly known as Web2.0 marketing and that is what we want to concentrate on.

Web 2.0 refers to a perceived second generation of web development and design, that facilitates communication, secureinformation sharing, interoperability, and collaboration on the World Wide Web. Web 2.0 concepts have led to the development and evolution of web-based communities, hosted services, and applications; such as social-networking sites, video-sharing sites,wikis, blogs, and folksonomies.”  Wikipedia – definition of Web2.0

So why is Web2.0 marketing important to market your business?  Well, where else can you be guaranteed to get such a huge audience for your company?  These social networking sites are used by millions of people, all around the world, every day. They really aren’t solely for young people eager to hook up with more ‘friends’ than anyone else.  These sites are home to millions of small communities of like-minded people, all of whom could be potential new customers.  
Now you can go straight in with a targeted advertising campaign that appears as a sponsored link to the side of peoples’ profiles, similar to Google AdWords, but this can be expensive and who has a large budget these days?  However, a much smarter way is to set up your own company profile, which is completely free.  Your profile then provides you with a platform to link to your own website and blog, which can build up interest in your products and services.  
For example, have a look at Virgin Media’s profile on Facebook and on Twitter
A few words of caution though.  To make Web2.0 marketing work for you requires you dedicate time to these sites.  You can’t simply put up a profile and expect anyone to visit you.  Nor should you go and alienate yourself by arriving with an enormous thump and selling hard.  Provide informative and interesting content.  Give honest opinions on your services and products. Appear credible!  
You must also be prepared for bad comments and unfavorable opinions.  Not everyone is going to agree with your nor are they going to like your products.  However, if you address each and every negative comment in a positive way, you will almost certainly make more friends and influence people!