This is the question that Nicholas Christakis and James Fowler address in their book, Connected: The Surprising Power of Our Social Networks and How They Shape Our Lives. From a Science + Religion Today article:
“In the book, we show that in real-world social networks, lots of things spread up to three degrees of separation, like weight loss, smoking cessation, binge drinking, happiness, loneliness, depression, piano teacher referrals, inventor ideas, generosity, and the list goes on and on. What this means is that if you make a positive change, it will influence your friends to do the same, but it doesn’t stop there—it also affects your friends’ friends, and even your friends’ friends’ friends. We call this the “Three Degrees of Influence” rule.
But not everything spreads in the same way. Emotions can spread quickly, even between people who do not have a strong relationship. [...] In contrast, behaviors seem to take longer to spread and they are only passed between people who have close social ties.”
The authors of the book also published a long article in The New York Times Magazine last year, entitled Are your friends making you fat?, in which they say your friends — and even your friends’ friends — can make you quit smoking, eat too much or get happy.
In other words: a look inside the emerging science of social contagion.