Communication activity in social networks: growth and correlations D. Rybski, S.V. Buldyrev, S. Havlin, F. Liljeros and H.A. Makse Eur. Phys. J. B, 84 1 (2011) 147-159 Published online: 26 October 2011 DOI: 10.1140/epjb/e2011-20172-4