Social-network research is of growing interest to Web developers, as the Internet moves from being an informational medium to a social forum filled with blogs, buddy lists, and instant messages.
Researchers are keen to develop models to explain the small-world phenomenon.
"The goal of our research was to try to understand what makes our world small," said David Liben-Nowell, a study co-author and computer science professor at Carleton College in Northfield, Minnesota. "We were trying to understand something about the why of the small-world phenomenon in a real network."
LiveJournal provided an ideal opportunity to study the friendship networks that people form, because the site allowed researchers to easily gather information about its users, including their interests, friends, and geographic locations.
"It became clear that geography was playing a major role in what was going on in the network," Liben-Nowell said. "Geography alone is essentially enough to explain why we live in a small world."
"If I have a message that I desperately need to pass along to [U.S. Vice President] Dick Cheney via a chain of intermediate friends, then it's a pretty good idea for me to initially send it to my high-school classmate Beth, just because she works in Washington, D.C.," he added.
From Idaho to Manhattan
The researchers found that in two-thirds of the friendships on LiveJournal, the friends shared a close geographic proximity. The other third of the relationships were "nongeographic," with friendships forming regardless of the distance between people.
Previous research has shown that friendships tend to decrease with distance. But the new study shows that the tendency also depends on population density.
Members living a mile apart would more likely be friends if they lived in Idaho than in Manhattan, researchers said.
"Consider my chances of being friends with a particular person—call him Bill," Tomkins said. "Draw a circle on the map, centered at me, with Bill on the outer edge. It's natural to think that my chances of being friends with Bill depend on how big the circle is, but that's wrong. Instead, it depends on how many people are in the circle."
The proper combination of distance between friends and density among them results in a "small world" phenomenon, the researcher added.
"If people make friends according to this [geographically] rank-based friendship, then short paths will magically emerge in the network, and people will be able to find them once they are forwarding messages," Tomkins said.
"But if you deviate a little bit from the formula—if you have too many [nearby] friends or too few [nearby] friends—then suddenly you won't have those short paths anymore and the network will become very difficult to navigate," he said.
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