1. A method of automatically partitioning users of a computer-based social network system into overlapping clusters, comprising: defining, using a computer system, connectivity matrices describing user connections and activities in the social network system, wherein the user connections and activities are derived from communications between the users; and analyzing eigenvectors and eigenvalues of the connectivity matrices to produce the overlapping clusters of users, wherein the eigenvectors and eigenvalues of the connectivity matrices are not principal eigenvectors and eigenvalues, and wherein the overlapping clusters are produced by selecting those clusters which exhibit maximum internal density and minimum external sparsity.