I've previously written on Mark Granovetter's classic study, The Strength of Weak Ties ("Weak Ties, Family Ties, and Business Success"). In it he found that found that people were far more likely to have used personal contacts in finding their present job than by other means, and of those personal contacts, most were what he called "weak ties." This led him to conclude that when it comes to finding jobs, our weak ties are often more useful than our strong ties. Why? Because our weak ties (i.e., our acquaintances) are less likely to be socially involved with one another than are our strong ties (i.e., our close friends).
Granovetter was recently awarded the Everett M. Rodgers for Achievement in Entertainment-Education (Rodgers, who died in 2004, was a sociologist who is best known for his book, The Diffusion of Innovations and for introducing the term, "early adopter."). Mark's acceptance speech has been captured on video (see below), and in it, he discusses, with his usual dry humor, some of the history lying behind his study, including Stanley Milgram's small world study. I highly recommend it.