Well, one reason may be that it is not as useful as it seems. On the other hand, the McKinsey Quarterly has some pointers for improving an organization’s use of social media as a marketing tool:
[Today’s] chief executive can no longer treat social media as a side activity run solely by managers in marketing or public relations. It’s much more than simply another form of paid marketing, and it demands more too: a clear framework to help CEOs and other top executives evaluate investments in it, a plan for building support infrastructure, and performance-management systems to help leaders smartly scale their social presence. Companies that have these three elements in place can create critical new brand assets (such as content from customers or insights from their feedback), open up new channels for interactions (Twitter-based customer service, Facebook news feeds), and completely reposition a brand through the way its employees interact with customers or other parties.
You can read the McKinsey post here.