I've just come across a paper that suggests employee who post negative comments about their employer can lead to positive outcomes.

There is a catch of course in that the ratio of "negative" postings to more positive postings should not generally exceed 15 to 20 per cent.

For more information about this paper - Blog, Blogger, and the Firm: Can Negative Posts by Employees Lead to Positive Outcomes? by Rohit Aggarwal - click here.