20. A computer readable memory device or storage device storing computer readable instructions that, when executed by one or more processing devices, cause the one or more processing devices to perform acts comprising: accessing records of messages sent by users; and based on the messages, computing an impact score for an individual user, the impact score being associated with a topic of the messages and reflecting power of the individual user to reach other users and authority of the individual user on the topic.
21. The computer readable memory device or storage device of claim 20 , wherein the computing further comprises computing the impact score from a power score indicating the power and an authority score indicating the authority.