Mark Granovetter is a sociologist who considered, among other things, a model of how fads are created.
Consider a hypothetical mob assuming that each person's decision whether to riot or not is depedent on what everyone else is doing. Instigators will begin rioting even if no one else is, while others need to see a critical number of trouble makers before they riot, too. This threshold is assumed to be distributed to some probability distribution. The fascinating thing in, that the outcomes may diverge largely although the inital condition of threshold may only differ slighlty.
Granovetter is furthermore well known by his classic article "The strength of weak ties" (in The American Journal of Sociology, Vol. 78, No. 6., May 1973), in which he argues that weak ties, e.g. the connections between colleagues, acquantainces etc., are more important for personal advancement, such as getting good jobs, than the strong ties of family and friendship.
He is also identified with the concept of embeddedness. This is the idea that economic relations between individuals or firms are embedded in actual social networks and do not exist in an abstract idealized market.
Related sociologists:
James S. Coleman
http://www.inequality.com/people/bios/granovetter.shtml
