A tiny fraction of those who use the fast-growing social network phenomenon Twitter generate nearly all the content, a Harvard study shows.

That makes it hard for companies to use the micro-blogging site as an accurate gauge of public opinion, the Harvard Business School study showed.

Twitter Inc is a social networking website in which users post messages of 140 characters or less -- known as tweets -- that can be viewed by other users who elect to follow them.

The Harvard study examined public entries of a randomly selected group of 300,000 Twitter users. The researchers studied in May the content created in the lifetime of the users' Twitter accounts.

It found that 10 percent of Twitter users generated more than 90 percent of the content, said Mikolaj Jan Piskorski, who led the research. More than half of all Twitter users post messages on the site less than once every 74 days.

The median number of lifetime tweets per user is just one, according the research.

Companies are increasingly turning to Twitter to improve their understanding of how consumers view them, he said.

But some users are far more active and vocal than others, limiting information gleaned from messages on the site, said Piskorski, an assistant professor of strategy at Harvard Business School.

If you're trying to get what a representative cross-section of the public is thinking, you're probably better off staying away from Twitter, he said in an interview.

Piskorski said Twitter could still be useful in responding to specific customer concerns. It can also be effective marketing, as companies with Twitter accounts can advertise sales and deals to users who follow them on the site.

JetBlue Airways Corp, Comcast Corp and Dell Inc are among companies with Twitter accounts.

JetBlue provides information on Twitter such as announcements of terminal changes at airports and tips on how to pack and directly answers customers' questions. Dell uses Twitter to point customers to discounts. Comcast's account largely responds to customer gripes.

Enough of our customers are talking about their interactions with us that we get a decent sample of what's going on a daily basis that we wouldn't necessarily get from other long-term studies, JetBlue spokesman Morgan Johnston said in a telephone interview.

Unlike other social networking sites like Facebook.com, men are almost twice as likely to follow other men on Twitter than they were to follow women, according to the study.

Women were also more likely to follow men than they were to follow other female users.

Piskorski said the study did not indicate whether this was because women were asked less often to be followed or because women are more selective in accepting followers.

Follow up research to these findings will likely be released later this month, he said.

According to recent data compiled by Nielsen Online, which measures Internet traffic, Twitter's website had more than 7 million unique visitors in February, compared to 475,000 in February 2008.

(Editing by Jason Szep and Eric Beech)

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