1. An information processing system comprising a microprocessor operating under the control of a computer program, the system comprising: an acquisition unit configured to acquire registrations of information related to provided information; a value identifying unit that sorts the registered related information based on chronological order of registration, groups the sorted related information into clusters of related information based on time interval durations between registrations of the sorted related information, and for each cluster, identifies a value of a parameter representing a registration frequency of the related information belonging to the cluster, and associates with the cluster a term appearing in the related information belonging to the cluster; a period identifying unit that identifies, for each cluster, based on the value of the parameter, a leading time period in which leading time period a ground swell of interest related to the provided information increases on average with a lapse of time, followed by an intermediate time period, followed by a tailing time period, in which tailing time period the ground swell of interest related to the provided information decreases on average with a lapse of time; and a feature identifying unit that identifies a leading post count indicating a number of posts made by each of the users during the leading time period, and ranks such leading post counts to categorize such users as pioneers of a trend associated with the term, an intermediate post count indicating a number of posts made by each of the users during the intermediate time period, and ranks such intermediate post counts to categorize such users as early adopters of a trend associated with the term, and a tailing post count indicating a number of posts made by each of the users during the tailing time period, and ranks such tailing post counts to categorize such users as followers of a trend associated with the term.