  The thermodynamic approach of concentration mapping is used to discover
communities in the directional friendship network of LiveJournal users. We show
that this Internet-based social network has a power-law region in degree
distribution with exponent \gamma = 3.45. It is also a small-world network with
high clustering of nodes. To study the community structure we simulate
diffusion of a virtual substance immersed in such a network as in a
multi-dimensional porous system. By analyzing concentration profiles at
intermediate stage of the diffusion process the well-interconnected cliques of
users can be identified as nodes with equal values of concentration.
