(CNN) -- Meet "network man." He has basic desires of his own, but has many arbitrary preferences, such as in music or clothes, that have been influenced by the people he knows.

Network man's likes and dislikes, in turn, affect the behavior of his friends, and their friends, and their friends. For example, when he gets into an obscure indie rock band, he shares an album with his friend, who likes it so much that he recommends it to his cousin, who spreads the word to her friends.

This is the view of human behavior put forth in "Connected," a new book by Dr. Nicholas Christakis, professor at Harvard University, and James Fowler, associate professor at the University of California, San Diego.

Examining years of research of their own and from others, the authors conclude that social networks, both offline and online, are crucial in understanding everything from voting patterns to the spread of disease.

People have profound influences on each other's behavior within three degrees of separation, the authors find. That means that your friends, your friends' friends, and your friends' friends' friends may all affect your eating habits, voting preferences, happiness, and more. At the fourth degree, however, the influence substantially weakens. Read about their research on how happiness is contagious

These are not small effects. "If a mutual friend becomes obese, it nearly triples a person's risk of becoming obese," the book said. Even geography doesn't matter; you're still at risk for gaining weight if a friend 1,000 miles away gets bigger. Listen to James Fowler talk about social networks »

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Who says click-through rates on Facebook suck?

Sure, click-through rates for general display ads on Facebook have been criticized for being rather unimpressive, but click-through rates for content on brand pages' walls are as high as 6.49%, according to estimates from Vitrue, a startup that helps marketers manage their social-media presences.

Earlier this week Vitrue announced a Social Relationship Manager suite with new planning and reporting tools for social media, including Facebook, where much of Vitrue's work is done. One of the things it has introduced is URL tracking, so it can measure click-through rates for links in wall posts and newsfeeds. Naturally, we wanted to find out what a typical click-through rate is for those messages.

Getting at the answer is a bit of science and a bit of guesswork, Vitrue acknowledged. That's because it's not always clear how many people are exposed to a link in a wall post, as it's syndicated out through newfeeds. In some cases people aren't online or on Facebook, which hinders total exposure to the message. To get at its click-through-rate estimate, Vitrue assumed that about one-twelfth of the Facebook audience is on the site at any given time and able to be exposed to a message. "We seem to feel comfortable it passes the sniff test," CEO Reggie Bradford said.

How many fans a brand has is also a factor in calculating click-through rate -- it's the total number of clicks on a particular post divided by number of fans who would have seen it, a number that's adjusted to take into consideration that not every fan is on Facebook all day long.

Mr. Bradford explained: "If a site has 100 fans and your wall post gets five clicks, that's a 5% CTR. But if you assume only about 20% of those folks actually saw the post, it's really a 20% click-through rate." That's better than the click-through rate of the average e-mail campaign, he said, and certainly better than the rate for an online ad. It also doesn't count how many people commented on the post or said they liked it but didn't click through.

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