Historically, and even now, service design has been all about understanding the motivations of the individual. Social network analysis on the other hand is based on the view that the attributes of individuals are less important than their relationships and ties with other actors within the network, and it is by exploiting these that you can really begin to focus on who's really important in your user network.
As an enterprise, social or otherwise, our users are most likely to belong to a random/exponential social network, but we want it to be a scale free network with hubs that positively reinforce our service. Now that we’re talking specifically about networks of people, this is where Tipping Point theory comes in. The tipping point is a concept related to collective behaviour and the fact that any behaviour pattern has a threshold at which point there is a sudden, marked and significant change. Tipping points are what makes marketing go viral; turning products into the epidemic type fads that companies dream about, but that’s a concept for another post.