When you think of Twitter and influence, you might think that the most obvious metric used to measure would be the number of followers a user has. Time and again, influence on Twitter has been shown to be not a direct function of how many followers one has, but a number of other factors.

One of those factors, according to a report by HP, may be just as obvious as follower numbers: long-standing status as a source of information and news. Having millions upon millions of followers may be fun, but it doesn't set the Trending Topics.

"Who gets to determine the big topics of conversation on social media? And how do they do it?" writes Ethan Bauley, managing editor of HP's Data Central blog.

According to Bernardo Huberman, director of HP Lab's Social Computing Research Group, it isn't the "most prolific tweeters or those with most followers" as you might expect.

"We found that mainstream media play a role in most trending topics and actually act as feeders of these trends," said Huberman. "Twitter users then seem to be acting more as filter and amplifier of traditional media in most cases."

According to Huberman's report, there are 22 Twitter users who dominate the Twitter Trending Topics. Bauley describes a bit of the work behind the report and its findings:

The HP team collected data from Twitter's own search API over a period of 40 days in the fall of 2010.  From the resulting sample of 16.32 million tweets, they identified 22 users who were the source of the most retweets when a topic was "trending."  Of those 22, 72% were Twitter streams run by mainstream media outfits such as CNN, the New York Times, El Pais and theBBC.

Although popular, most of these sites have millions of followers fewer than highly followed tweeters such as Ashton Kutcher, Barack Obama or Lady Gaga.

Similarly, the research showed that just having an active Twitter account was not a factor in creating a trend.

What were these 22 accounts? Take a look.

For the intellectually curious, the report is embedded blow in its entirety.

Trends in Social Media: Persistence and Decay