You and Your Friend’s Friend’s Friends

"...we are part of a superorganism, a hivelike network that shapes our decisions. “A smoker may have as much control over quitting as a bird has to stop a flock from flying in a particular direction...."

Here is one more excerpt:
"How does network contagion work?.....Partly, it’s a kind of peer pressure, or norming, effect, in which certain behaviors, or the social acceptance of certain behaviors, get transmitted across a network of acquaintances."

 And there is explanation using evolution, "During the early stages of human evolution, selective advantage was probably conferred on those individuals who lived in social networks and could share information about food or predators. The primatologist Robin Dunbar has argued that the human brain evolved to its present size to keep track of a network of 150 people..........As among primates, those humans who are best able to manipulate social networks to their advantage thrive, and that ability may be genetically encoded........."

Though it has plenty of assumptions, this is an article to read.