C. Lampe, N. Ellison, and C. Steinfield. Proceedings of the 2006 20th anniversary conference on Computer supported cooperative work, page 167--170. New York, NY, USA, ACM, (2006)
M. Morita, and Y. Shinoda. 17th Annual International ACM SIGIR Conference on Research and Development in Information Retrieval, page 272--281. Springer-Verlag New York, Inc., (1994)