While my knowledge of the social media landscape is limited, it seems to me like many businesses have really latched onto Twitter as a mainstay in communicating with their customers.  In some cases, technology-centric companies have shifted an enormous amount of their communication onto the service, often at the expense of other, traditional means ('ancient' formats like email newsletters and user forums).

I don't have first-hand experience with how well businesses fare on Facebook, but that always seemed like an odd fit to me.  I guess it depends on how open users are to broadcasting their purchasing habits and product-preferences to their friends and acquaintances.  On the other hand, I've had very positive conversations with co-workers and (especially) bosses in regards to LinkedIn, and the benefits of getting into circles of "who you know".   I wonder if this works better due to the focus on separating professional life, rather than attempting to blend users' personal and professional lives on the same platform.