Check out this chart from www.mashable.com below which provides survey results of the benefits companies are seeing in social marketing. I think it provides a great overview of the potential for social marketing. The only thing that I would add is the value of connecting with influencers (call it indirect communications) to reach a broader audience is not captured. I would add that as a category the next time they run this survey. I think they will be surprised how well this category results.
Also, social marketing is not just about social networking, but developing an online community into your existing website, building social profiles into your CRM efforts, and extending your multi-channel communications. Finally, it is about rethinking the customer experience to better orient around the 360 degree referrential buying process this is becoming the norm.