1. Institute of Sociology and
2. Center for Sociological Research, University of Leuven, Leuven B-3000, Belgium;,
3. URPP Social Networks, University of Zurich, Zurich CH-8050, Switzerland;
4. University of Finance and Management in Warsaw, Warsaw 01-030, Poland;
5. National Research University Higher School of Economics, Moscow 101000, Russia
6. Department of Political Science, Justus Liebig University Giessen, Giessen D-35394, Germany;