We've all heard it: The Internet has flattened the world, allowing social networks to spring up overnight, independent of geography or socioeconomic status. Who needs face time with the people around you when you can email, text or tweet to and from almost anywhere in the world? Twitter, the social networking and microblogging site, is said to have more than 300 million users worldwide who follow, forward and respond to each other's 140-character tweets about anything and everything, 24/7. But MIT researchers who studied the growth of the newly hatched Twitter from 2006 to 2009 say the site's growth in the United States actually relied primarily on media attention and traditional social networks based on geographic proximity（亲近，接近） and socioeconomic similarity. In other words, at least during those early years, birds of a feather flocked -- and tweeted -- together.

In their study of Twitter's "contagion（蔓延，传染） process," the researchers looked at data from 16,000 U.S. cities, focusing on the 408 with the highest number of Twitter users and seeking to update traditional models of how information spreads and technology is adopted.

Just as marketing experts sometimes label consumers as early adopters, early majority adopters, late majority adopters or laggards（落后者） , the researchers characterized cities in those terms, based on when Twitter accounts in a given city reached critical mass. Critical mass is generally defined as the point when something reaches 13.5 percent of the population, which for this study was 13.5 percent of the highest total number of Twitter users in a city through August 2009, the end of the study period.

As with most technologies, the growth in popularity initially spread via young, tech-savvy "innovators," in this case from Twitter's birthplace in San Francisco to greater Boston. But the site's popularity then took a more traditional route of traveling only short distances, implying face-to-face interactions; this approach made early adopters of Somerville, Mass., and Berkeley, Calif. -- cities close to Boston and San Francisco, respectively. Twitter use then spread through early majority cities such as Santa Fe and Los Angeles and late majority cities such as Baltimore and Las Vegas before reaching laggards such as Palm Beach, Fla., and Newark, N.J. All these cities ultimately ranked among the 408 nationwide with the largest numbers of Twitter accounts.

"Even on the Internet where we may think the world is flat, it's not," says Marta González, assistant professor of civil and environmental engineering and engineering systems at MIT, who is co-author of a paper on this subject appearing this month in the journal PLoS ONE. "The big question for people in industry is 'How do we find the right person or hub to adopt our new app so that it will go viral?' But we found that the lone tech-savvy person can't do it; this also requires word of mouth. The social network needs geographical proximity. … In the U.S. anyway, space and similarity matter."