http://www.boxesandarrows.com/view/social-networks
Granovetter (1973) argued that within a social network, weak ties are more powerful than strong ties. He explained that this was because information was far more likely to be “diffused” through weaker ties. He concluded that weak ties are “indispensable to individuals’ opportunities and to their incorporation into communities while strong ties breed local cohesion.”
Granovetter’s doctoral thesis demonstrated that most people landed jobs thanks to their weak ties and not their strong ones. It was the people that they did not know well, the ones with whom they did not have shared histories and did not see on a regular basis who were of most help. This is because people with strong ties generally share the same pieces of information and resources. Therefore they are of less help to one another.