You wouldn’t walk up to a co-worker’s cubicle and challenge him to do 25 sit-ups on a typical workday, but you might challenge him online if your company was using one of the new social media platforms designed to encourage employees to stay (or get) in shape.  Or you might find that challenge in your own inbox, or an offer to go for a bike ride after work.

A growing number of companies are banking on social media to boost the participation rates in their employee wellness programs. As it is, about 90 percent of companies (with more than 5,000 employees) use the web to deliver their wellness programs. For example, one employee wellness company, ShapeUp, has a Facebook-like platform, where people invite “friends” to participate, create “teams,” and can log their own fitness and weight control efforts and see how they’re doing compared to others in their company.

“The prime motivator is the social accountability we engender,” said Dr. Rajiv Kumar, founder and chief medical officer of ShapeUp. The “friends” can be a powerful motivating force, especially when everyone can see how much (or little) you’ve done each week.

“We believe this peer accountability, which is stronger than accountability to a faceless HR department, can be as powerful, if not more, than financial incentives,” Kumar said.

According to ShapeUp’s research, about 30 to 50 percent of employees at their client companies participate in their social media wellness program compared to, say, a typical walking program, for which 15 percent participate, or a weight loss program, for which about 8 percent partake.

Other studies explain why social media may help motivate people to exercise more or lose more weight. These studies show that peers have a big impact on your health behaviors. When people are losing weight around you, you’re more likely to lose weight, and when they’re quitting smoking, you’re more likely to quit too. But it can also go the opposite way - when they eat donuts, you’re likely to do that as well!   

ShapeUp makes fitness and nutrition challenges and the teams compete against each other to see who can walk the furthest (measured with pedometers), bike the most miles, lose the most weight, eat the healthiest, and do the most sit-ups. Employees log their efforts and accomplishments daily or weekly. If you don’t enter your log for a few days, you may get a friendly nudge from a teammate. On the other hand, when you run a 5K or simply go for a long walk, you might get a round of cyber high-fives from your teammates.

One of the best aspects of these social media wellness programs is that they are like Match.com for exercisers. You can crowd source a cycling partner, jogging buddy or someone to play tennis with.  If your company isn’t doing social media wellness, you can check out Fitocracy and Daily Mile, direct-to-consumer websites that use the same concept of social media to help people meet their fitness and nutritional challenges.

Laurie Tarkan is an award-winning health journalist whose work appears in the New York Times, among other national magazines and websites. She has authored several health books, including "Perfect Hormone Balance for Fertility." Follow her on Twitter and Facebook.