Jyri Engeström's weblog has many interesting posts; this one, from April 2005, about social networks brings up particularly relevant approaches to analysis of the topic. He uses the concept of "object-centered sociality" as well as actor-network theory to look at the development, and potential for demise, of social networks.

"...the term 'social networking' makes little sense if we leave out the objects that mediate the ties between people. Think about the object as the reason why people affiliate with each specific other and not just anyone. For instance, if the object is a job, it will connect me to one set of people whereas a date will link me to a radically different group."