From the abstract of this HP Labs research paper:
In this paper, we conduct an intensive study of trending topics on Twitter and provide a theoretical basis for the formation, persistence and decay of trends. We also demonstrate empirically how factors such as user activity and number of followers do not contribute stronglyto trend creation and its propagation. In fact, we find that the resonance of the content with the users of the social network plays amajor role in causing trends.
The last point is the main thing I wanted to see elaborated. I'm not sufficiently bothered about this area to get to grips with the maths in the middle of the paper, so I just skipped to the Trend-Setters and Conclusions sections. The researchers explain how they identified Twitter users who initiate the largest number of trending topics, and the process of propagation, mainly through retweeting.
From the conclusion:
When we considered the impact of the users of the network, wediscovered that the number of followers and tweet-rate of users are not the attributes that cause trends.
So it's not that there's a master-race of Influentials who call the tune that the masses dance to.
What proves to be more important in determining trends is the retweets by other users, which is more related to the content that is being shared than the attributes of the users.
The message is important, and some messages resonate more than others. There's a hint here, which Mark Earls I'm sure would want to draw out, that being able to see what resonates with others could be a major factor in amplifying one's sense of how important an issue is, and may influence one's likelihood of retweeting it oneself. A herd effect.
Finally,
Furthermore, we found that the content that trended was largely news from traditional media sources, which are then amplified by repeated retweets on Twitter to generate trends.
So Twitter hasn't completely usurped the usefulness of mainstream media, and the two coexist in a new ecology. Big surprise.