Here we're compiling some papers on friendship networks.
Community structure and ethnic preferences in school friendship networks by M. C. Gonzalez, H. J. Herrmann, J. Kertesz, T. Vicsek (physics/0611268)
Rhythms of social interaction: messaging within a massive online network by Scott Golder, Dennis M. Wilkinson, Bernardo A. Huberman
A Gap in the Community-Size Distribution of a Large-Scale Social Networking Site by Kikuo Yuta, Naoaki Ono, and Yoshi Fujiwara
Finding Community Structure in Mega-scale Social Networks by Ken Wakita and Toshiyuki Tsurumi (like the above paper, the data they use is from mixi, a social networking service in Japan)