A media research firm called CARMA International has released a survey in which they found that bloggers tend to loathe Walmart and adore Costco—not a surprise. Bloggers, they found, also tend to love Target and hate Macy’s.

CARMA monitored some 50 million blogs every day during January and February, looking for mentions of 17 different retailers. They examined 3,700 blogs that discussed those retailers. According to CARMA, “Nearly 60 percent of all blog posts on wholesale and club stores were favorable, while only 9 percent of posts were unfavorable.” Mass merchandisers, by contrast, received mostly negative commentary. Walmart was often described as “evil.”

CARMA’s Christopher Scully, a VP and author of the study, was surprised: “We thought Walmart's financial coverage in the mainstream media would foster positive attention overall from the blogosphere, but that wasn't the case,” he said. “Bloggers criticized the company severely for being anti-worker, driving mom-and-pop stores out of business, and contributing to sprawl and to the decline of the American manufacturing base, and this hurt its overall depiction in blogs.”

What this means, Scully said, is that while the news can be treating your company well, public opinion can still be low. He recommends big companies monitor social media as well as news sources.

Bloggers and letter writers can be a powerful force. Take the case of Tropicana orange juice: Pepsi, the parent company of Tropicana, redesigned the packaging in early January, only to revert to the old straw-sticking-out-of-an-orange design the next month.

The good news, as one public relations expert told the New York Times in February, is that “Twitter is the ultimate focus group”—in other words, companies can learn a lot from social media without having to organize focus groups themselves. But the bad news is that the response can be quick and vicious and the demographics are nearly impossible to pin down.

Neil Campbell, president of Tropicana North America admitted that his company had “underestimated the deep emotional bond” consumers had with the original design. But he also acknowledged that it came from “a fraction of a percent of the people who buy the product.”

Then again, unit sales reportedly dropped some 20 percent and competitors’ products saw a jump in sales after the straw-in-orange design was taken away. This is the real reason Tropicana listened to the criticism.

Should we all be paying so much attention to bloggers? Really loud voices only indicate a depth of passion, not necessarily a widespread truth.

The other problem with listening to those loud voices is that people seldom make extraordinary efforts to shout about how content they are with your company and its services. When things go the way they should typically go—when consumers are pleased with a company’s ads, logos, branding, customer service, community interaction and financial performance—they probably aren’t rushing to their computers to blog about it.

The lesson here is that while bloggers can tell you what you’re doing wrong, they don’t often reward you for what you’re doing right. What’s more, they often only represent either your most loyal and passionate customers (in the case of Tropicana) or people who may never shop at your store anyway (in the case of Wal-Mart). Analyze such negativity with care.

[To see a PDF of the CARMA study, click here.]