1. A computer-implemented method for use with data identifying a plurality of users and a plurality of achievements, the data further identifying which of the plurality of achievements each of the plurality of users has earned and when any such achievements were earned, the method being performed by at least one computing device, the method comprising: ordering the plurality of achievements to define an ordered achievement list; for each of the plurality of users, removing from the ordered achievement list any of the plurality of achievements identified in the data as having been earned by the user to define a recommended achievement list for the user; identifying a selected achievement; and identifying as a set of similar users any of the plurality of users whose recommended achievement list includes the selected achievement.