The National School Boards Association (USA), in partnership with research firm Grunwald Associates and with the support of Microsoft, Newscorp, and Verizon, published a 2007 data-rich survey dissecting social and education related activity patterns by American students. The focus of the study was K-12 instruction; however, much can be learned from the results of this study that apply to the uses of social networking technology in general.

Ruth Reynard, "Social Networking: Learning Theory in Action," Campus Technology, 5/28/2008.
Thanks to Chuck Ferguson for mentioning this article.