Many products live or die by word of mouth, but marketers have never been able to fully measure what the public thinks outside of limited focus groups and surveys. Increasingly, they're turning to Weblogs, online discussion groups and product review sites to hear what consumers are saying. One of the companies they hire for such research is BuzzMetrics , which counts dozens of Fortune 500 corporations among its clients. This week the company released a study which monitored online consumer discussions about the dangers of trans-fat in foods. BuzzMetrics says those discussions are leading the packaged food industry to change the way they manufacture and market food.