On a related note, today's episode of Rocketboom was entitled The Social Construction of Reality which is taken from the name of a 1966 book by Peter Berger and Thomas Luckmann. The book sounds like it might have some applicability to how social networks are formed.
The work introduced the term social construction into the social sciences. The central concept of The Social Construction of Reality is that persons and groups interacting together in a social system form, over time, concepts or mental representations of each other's actions, and that these concepts eventually become habituated into reciprocal roles played by the actors in relation to each other. When these roles are made available to other members of society to enter into and play out, the reciprocal interactions are said to be institutionalised. In the process of this institutionalisation, meaning is embedded in society. Knowledge and people's conception (and belief) of what reality is becomes embedded in the institutional fabric of society. Social reality is therefore said to be socially constructed.