J. Zhang, M. Ackerman, and L. Adamic. WWW '07: Proceedings of the 16th international conference on World Wide Web, page 221--230. New York, NY, USA, ACM, (2007)
L. Adamic, J. Zhang, E. Bakshy, and M. Ackerman. WWW '08: Proceeding of the 17th international conference on World Wide Web, page 665--674. New York, NY, USA, ACM, (2008)