Letierce, Passant, analysed Twitter feeds from the International Semantic Web Conference (#iswc2009), the Online Information Conference 2009 (#online09) and the European Semantic Technology Conference (#estc2009). First, they checked the distribution of tweets per user, then the distribution of tweets that were directed to individuals (@user messages). They found that both were Power Law distributions.
After that, they used the HITS (Hyperlink-Induced Topic Search) algorithm to determine the hubs and authorities of the conference feeds. Users who addressed many @user messages were considered hubs, while the users who received many @user tweets were considered authorities. Letierce et al.'s not-very-surprising conclusion was that users with both high hub and authorities scores were often the organizers of the events studied in the research. Also, users with real-world authority (their example was @timberners_lee) also had t-authority. Of course, Letierce and her colleagues couldn't determine if there are more real-world authorities that don't use Twitter (perhaps a content analysis of the tweets can determine if there are talks about people who aren't Twitter users, but that's very time-consuming and not the point of the research here).
In addition, the paper includes a small survey (61 participants) conducted by the authors, but I chose not to discuss it here, because of the small sample.
Letierce, J., Passant, A., Breslin, J., & Decker, S. (2010). Understanding how Twitter is used to spread scientific messages Proceedings of the WebSci10: Extending the Frontiers of Society On-Line, April 26-27th, Raleigh, NC: US.