Via Malcolm Gladwell’s excellent book The Tipping Point: How Little Things Can Make a Big Difference:

In his classic 1974 study Getting a Job, Granovetter looked at several hundred professional and technical workers from the Boston suburb of Newton, interviewing them in some detail on their employment history.He found that 56 percent of those he talked to found their job through a personal connection. Another 18.8 percent used formal means — advertisements, headhunters — and roughly 20 percent applied directly. This much is not surprising; the best way to get in the door is through a personal contact. But curiously, Granovetter found that of those personal connections, the majority were “weak ties.” Of those who used a contact to find a job, only 16.7 saw that contact “often” — as they would if the contact were a good friend — and 55.6 percent saw their contact only “occasionally.” Twenty-eight percent saw the contact “rarely.”People weren’t getting their jobs through their friends. They were getting them through acquaintances.

Why are people you’re less close to more valuable in terms of finding a job? You’re more likely to know the same people and things your good friends do.

Acquaintances “give us access to social networks where we don’t otherwise belong.”