Massachusetts Institute of Technology (MIT) researchers studied the growth of Twitter from 2006 to 2009 and found that its initial growth in the United States relied heavily on media attention and traditional social networks based on geographic proximity and socioeconomic similarity. The researchers examined data from 16,000 U.S. cities, focusing on the 408 with the highest number of Twitter users. They characterized cities as early adopters, early majority adopters, late majority adopters, or laggards based on when 13.5 percent of the population had Twitter accounts. "The social network needs geographical proximity," says MIT professor Marta Gonzalez. "In the U.S., anyway, space and similarity matter." The researchers found that Twitter's initially popularity was fueled by young, tech-savvy users in San Francisco and Boston. However, after that it began to follow a more traditional pattern by traveling short distances, which implied that personal interactions were important to its wider use. "Nobody has ever really looked at the diffusion among innovators of a no-risk, free, or low-cost product that's only useful if other people join you," says MIT graduate student Jameson Toole. "It's a new paradigm in economics: what to do with all these new things that are free and easy to share."