S. Wu, J. Hofman, W. Mason, and D. Watts. Proceedings of the 20th international conference on World wide web, page 705--714. New York, NY, USA, ACM, (2011)
H. Kwak, C. Lee, H. Park, and S. Moon. WWW '10: Proceedings of the 19th international conference on World wide web, page 591--600. New York, NY, USA, ACM, (2010)