Can the Web make people more fit? It’s a question hot on the minds of everyone from health insurers to gym owners to public health officials. Although millions of dollars a year are being spent designing promotional ads and social media campaigns, they clearly aren’t working: more than 43 percent of Americans get insufficient levels of daily exercise, and nearly a third are obese. In a new study, researchers at the University of Pennsylvania, led by professor  Damon Centola  of the Annenberg School for Communication and the School of Engineering and Applied Sciences, have found a way to make the Web — and social media — more effective for improving people’s exercise habits. The study,  recently published in the journal  Preventive Medicine Reports , tested a fitness motivator that can be more effective — and vastly cheaper — than promotional advertisements:  program-assigned “health buddies.” In a randomized controlled t...