"It’s really unlikely that A knows C and A knows B but B and C don’t know each other, at any rate if A is pretty good friends with both B and C: B and C will probably know each other too.

If A needs a job, she’ll ask B and C. They probably won’t have any new information, because A already shares most of the information that B and C have. There’s a far greater chance A will get new information — for instance about a job that might suit A — from her weak ties, that is from aquaintances and people that she doesn’t see very often. The greater social distance between A and D means that D knows more things that A doesn’t already know.

Weak ties also important because they work as bridges between social groups. People who are bridges between two groups may appear to be socially isolated but actually have weak ties with two or more groups which gives them very early access to new information.