In a previous post I mentioned Do The Right Thing - a site that lets people vote on the social impact of corporations. There seems to be a conversation going on as to the value of this site, in the sense that it would create a bias towards those companies that have really good PR. More on Marginal Revolution here. This is a tricky question, because impact might be in the eye of the beholder. I think if we define the impact as the public's perception of impact, from news published by the company, and by articles in the media (and also and especially blogs), then the real message to companies would be to communicate. And then up to the public to keep them honest - in a "say what you do - do what you say" mode.