Traditional carpet bombing advertising does not work well in social media networks. Heavy PR (one on one) conversational marketing is too costly. Your spending too much money, time and resources to get to the influencers. Brands who want to be effective with their scarce resources (budgets & marketing department personal), NEED to find out who is the top 5-10 influencer in their market and focus their one on one (conversational) marketing primarily on them.
A few examples of brands & Influencers taking advantage of Twitter: Shaq (THE_REAL_SHAQ), Lance Armstrong (lancearmstrong), MC Hammer (MCHammer), Walt Mossberg (waltmossberg). Get to these guys be it product reviews or endorsements, the larger the social media ROI. Have you seen how many "followers" these guys have on Twitter? It’s ridiculous.