Our resident social media guru at ddm, Paul Kortman, passed along an interesting article that attempts to categorize an assortment of social media tools according to their most effective marketing applications. I agree with many of the conclusions of the article, so here is a link for your edification:
http://www.seomoz.org/blog/a-checklist-to-choose-which-internet-marketing-channel-is-right-for-your-business
Mike