A system, apparatus, computer-program product and method are provided for identifying members of a particular user segment. One particular user segment of interest includes members who work for employers having a number of employees within a predetermined range. Members of an online service provide data purporting to identify their employers and/or other personal or professional attributes. The data entries are normalized by standardizing terms, removing superfluous or unneeded terms, and/or performing other processing. The members are then clustered according to their normalized employer names, and members within clusters that have sizes within the range are added to the user segment. Invalid employer names (e.g., fictitious companies, non-existent entities) may be filtered out. Within a cluster, a professional social networking site (or other site) may be analyzed to determine if the clustered members have developed relationships; if not, the cluster may be cancelled.