The number one benefit of social media marketing is the lower cost of entry, when compared to more traditional marketing techniques. Essentially, you are building on the back of your own network of contacts and clients, using that social network to further your campaign.
The tools most associated with social media (Twitter, Facebook, MySpace etc) are, for the most part, free. However, social media marketing is still marketing. You will still need the more traditional spread of tools to measure and analyze your activities.