The U.S. Defense Advanced Research Projects Agency's (DARPA) Network Challenge earlier this month demonstrated that social networks, more than being platforms for self-promotion, can be also be highly effective tools for rapidly gathering and disseminating very precise information. With the help of Facebook, Twitter and a homemade Web site, a winning team from the Massachusetts Institute of Technology (M.I.T.) was able to within nine hours identify the correct latitude and longitude of all 10 of DARPA's red weather balloons, which were lofted 30.5 meters into the air at locations scattered throughout the U.S.

The competition was instructive not only in the ways that social networks can be successful at crowd-sourcing—using the Internet's long reach to rally disparate groups of people together for a common cause—but also in revealing different ways people can be motivated to participate. Whereas the M.I.T. Media Lab's Red Balloon Challenge Team's approach was to promise a share of the $40,000 in prize money to those that helped them win, the Georgia Tech Research Institute (GTRI) "I Spy a Red Balloon" team in Atlanta (which placed second out of 58 teams) located nine of the balloons in nearly the same amount of time as M.I.T. with the promise of donating all of their winnings to the American Red Cross.

The M.I.T. researchers jumped at the opportunity to participate in a competition that would allow them to further their Human Dynamics Laboratory studies of how people interact with technology. Almost from the outset the team decided on a "temporary recursive incentive" model that would reward not just the balloon finders but also the network of people responsible for putting the team in touch with the balloon finders, says Wei Pan, a first-year M.I.T. Media Lab graduate student and member of the winning team.

The Red Balloon Challenge Team offered $4,000 in reward money per balloon. This included $2,000 per balloon to the first person to send them the correct coordinates and $1,000 to the person who invited the balloon finder to join M.I.T.'s balloon-finding network. If there were a third person in the network chain (the person who invited the inviter to join), that person would receive $500. Whoever referred the $500 winner to the team would be entitled to $250, and so on. The team—which included students Pan, Manuel Cebrian, Anmol Madan, Galen Pickard and Riley Crane as well as Human Dynamics Laboratory director Sandy Pentland—decided any leftover money would go to charity.

"We wanted to understand how Facebook, Twitter and other social networks have changed the way humans mobilize," Pan says. "We also wanted to see how, with minimal cost to us, we could harness the powerful resources offered by the online community." The balloons ended up being widely dispersed throughout the U.S., in cities including Memphis, Miami and San Francisco as well as smaller venues such as Christiana, Del., and Katy, Tex.

The GTRI team considered paying for reported information about the balloons' locations but opted against it. "We figured we could get our network more widespread if we appealed to them by supporting a charity," says Erica Briscoe, a GTRI research scientist. The team carefully considered charities, ultimately choosing the American Red Cross. "We were looking for something nonreligious and noncontroversial," says GTRI research engineer Ethan Trewhitt. "We didn't want anyone using the charity as a reason for not joining."

GTRI's first order of business was to create a Web site and a group within the Facebook community. The team also signed up for a G-mail and Google voice mail accounts to be used exclusively for people looking to contact them with information during the competition. Team members deluged their friends with Facebook invitations and encouraged those friends to invite their friends. GTRI issued a press release via its Web site and announced the team's entry via Twitter. The team's Web site received about 3,000 unique visitors prior to competition, and their Facebook group had 850 members.

To ensure that the GTRI team's Web site would appear near the top of any Google search related to the DARPA Network Challenge, Briscoe joined online discussion forums, commented on Web sites where the competition was discussed and did an interview with National Public Radio, being careful to emphasize that any prize money would go to charity.

Both M.I.T.'s and GTRI's teams needed to be on guard against being duped by fake weather balloon sightings. M.I.T. did this in part by checking the IP addresses of e-mails that claimed to be providing accurate coordinates. If the IP address identified a computer nowhere near the balloon that the e-mailer was claiming to have found, the message was looked on with suspicion. M.I.T. also asked its informants to e-mail images of the balloon they had found and/or the image of a certificate obtained from the DARPA officials stationed at each balloon site. These measures proved useful in debunking a report from a supposed informant on the campus of Brown University in Providence, R.I. After analyzing the image sent by the informant, the M.I.T. team determined that the picture was actually a copy of one from DARPA's Network Challenge Web site, doctored to look as though the balloon was hovering over Brown.

The GTRI team also received the Brown University report, which they likewise dismissed as a fake. In addition to studying e-mailed images, GTRI called friends, family and local businesses to help validate alleged sightings. They sent two team members out to Atlanta's Centennial Olympic Park to confirm reports of a balloon there. Sure enough, the investigators returned with a certificate from DARPA authenticating the find.