Some unassailable insights here from McKinsey and Co.:
When you think of word of mouth as media, it becomes a form of content, and businesses can apply tried-and-true content-management practices and metrics to it. In addition, word of mouth generated by social networks is a form of marketing that must be earned—unlike traditional advertising, which can be purchased. We therefore concluded that we could succeed only by being genuinely useful to the individuals who initiate or sustain virtual world-of-mouth conversations.
There’s lots more good stuff in there, too, so I’d strongly suggest you read the whole piece.
Unlocking the elusive potential of social networks (via McKinsey Quarterly)