A recent great study by Yang and Leskowec at Stanford University titled Modeling Information Diffusion in Implicit Networks provides an underlying model and brings up some interesting findings
Findings include that professional blogs are the most influential news media in sports and the second most influential media in politics and national news, while personal blogs are the most influential in entertainment and the second most influential in technology. In general the influence of blogs tends to decay more slowly than other media.
One of the most intersting conclusions is that the most visible agents may not be the most influential when it comes to “social ” phenomena such as hashtags.
“As the adoption of short, news-related textual phrases appears active users, each of which has relatively less influence. Moreover, we also observe that users with the most followers are not the most influential in propagating hashtags.”