The mainstream media play a role in most trending topics on twitter

"In a new paper, Huberman and three fellow researchers demonstrate that “user activity and number of followers do not contribute strongly to trend creation and its propagation.”
Instead, says Huberman, “we found that mainstream media play a role in most trending topics and actually act as feeders of these trends.  Twitter users then seem to be acting more as filter and amplifier of traditional media in most cases.”
How trends start
The main determinant of whether an item trends – much more than who tweets about it or how often – was the specific subject of the tweet, the team found. That can be seen in the degree to which trending topics are the result of retweets – items passed from one individual’s network of followers to another’s.  31% of tweets of trending topics are retweets, their analysis showed.
The role of “traditional” media sources
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 the BBC.
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."
Source:  White Paper from HP, 12th February 2011