This is an attempt to define user visibility on a specific topic. Briefly, tweets are collected via the Twitter streaming API, stored in sqlite databases and then processed in order to create a regular Markov chain. The steady state distribution of the chain defines a metric on the set of Twitter users, which can be used to retrieve an ordered list of users.
Have a look at this paper and this other paper for further details about the mathematical methods.
Be careful, the procedure described here is experimental and it is not meant to be used in production environments.