Yahoo! Research has released Who Says What to Whom on Twitter.
Here's an excerpt:
We study several longstanding questions in media communications research, in the context of the microblogging service Twitter, regarding the production, flow, and consumption of information. To do so, we exploit a recently introduced feature of Twitter—known as Twitter lists—to distinguish between elite users, by which we mean specifically celebrities, bloggers, and representatives of media outlets and other formal organizations, and ordinary users. Based on this classification, we find a striking concentration of attention on Twitter—roughly 50% of tweets consumed are generated by just 20K elite users—where the media produces the most information, but celebrities are the most followed. We also find significant homophily within categories: celebrities listen to celebrities, while bloggers listen to bloggers etc; however, bloggers in general rebroadcast more information than the other categories. Next we re-examine the classical "two-step flow" theory of communications, finding considerable support for it on Twitter, but also some interesting differences. Third, we find that URLs broadcast by different categories of users or containing different types of content exhibit systematically different lifespans. And finally, we examine the attention paid by the different user categories to different news topics.