The Strength of Weak TiesIn 1973 Mark Granovetter, a sociologist now at Stanford, published a paper, "The Strength of Weak Ties" (pdf),1 in which he distinguishes between strong ties between individuals, i.e., relationships in which the individuals are friends, and weak ties, i.e., relationships in which the individuals are mere acquaintances.
Granovetter then goes on to argue, "Weak ties are more likely to link members of different small groups than are strong ones, which tend to be concentrated within particular groups." The significance of this is that weak ties can serve as bridges between social networks, and thereby give individuals in one network access to information beyond what is already known amongst their friends.
For instance, if someone is looking for a job, tapping acquaintances can provide additional leads beyond those that friends may be aware of. Or, if a community is trying to organize for collective action (Granovetter uses the example of threatened destructive urban renewal), it will have more success if weak ties facilitate the uniting of multiple closely knit networks within the community.
In a business setting, weak ties that act as bridges are valuable in any situation in which tapping expertise outside a team will help the team accomplish more, do a better job, and/or achieve results faster.
In the early '80s, Granovetter undertook a review of empirical studies testing the hypotheses in his 1973 paper. The review was published in final form in 1983, and you can read it here.2
1 Mark Granovetter, "The Strength of Weak Ties," American Journal of Sociology, Vol. 78, No. 6 (May 1973), pp. 1360-1380.
2 Mark Granovetter, "The Strength of Weak Ties: A Network Theory Revisited," Sociological Theory, Vol. 1 (1983), pp. 201-233.