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Who determines the big topics of conversation on social media such as Twitter? It’s isn’t the loudest bloggers with large followings of virtual fans.

Rather, mainstream media tends to play the biggest role in the top “trending” topics on Twitter, or those that appear in the top ten search terms on the service at any given point in time, according to a study by Hewlett-Packard’s research labs.

“You might expect the most prolific tweeters or those with most followers would be most responsible for creating such trends,” says Bernardo Huberman,  HP Senior Fellow and director of HP Labs’ Social Computing Research Group.

That turns out not to be the case, Huberman and three other researchers found. User activity and the number of followers do not contribute strongly to trend creation and its propagation. Rather, Twitter users seem to be acting as a filter and amplifier for the trends that are written about by mainstream media. The results are not so different from those shown by Technorati, the blog-tracking search site that records which mainstream media sites get the most links.

The main determinant of a fast-growing topic on Twitter is the specific subject at hand, not who tweets it or how often. About 31 percent of trending topics are retweets, or someone passing the tweet on to their friends.

The HP team drew data from Twitter’s own search applications programming interface (API) over of 40 days in the fall. Analyzing 16.32 million tweets, they found 22 users who were the source of the most retweets when a topic was “trending.” Of those, 72 percent were Twitter streams run by mainstream media outfits such as CNN, the New York Times, El Pais and the BBC. Most of these sites have fewer followers than Twitter celebrities such as Ashton Kutcher, Lady Gaga or Barack Obama.

Very few topics stay much more than 40 minutes at the top. But those that do are engaging to a very diverse audience.

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