Researchers found that the prospect of workout goals being shared on social media led fewer people to set weekly goals.
At the same time, the sharing led to emotional encouragement, logistical assistance and accountability that may have helped study participants, who all were classified as obese, up their step counts by an average of about a half mile per day.
"One of the most common recommendations for people who want to lose weight is to share their plan with family and friends. The theory is that being accountable to, and encouraged by, their social support system will increase their chances of reaching goals," said Paul Resnick, the Michael D Cohen Collegiate Profession of Information at the University of Michigan.
But the researchers at the University of Michigan and University of Washington found that sharing was a two-edged sword.
"Public accountability is great, but not if it keeps you from making commitments in the first place," Resnick said.
In the 12-week, randomised, controlled clinical trial, the researchers gave 165 people classified as obese FitBit pedometers and access to a custom website they had created.
At the website, participants could view their steps and choose whether to set a step goal for the following week.
The study put the participants in three categories, and people knew which one they were in. For one group, goals and results (whether they met the goal) were kept private.
In another, any goals the participants made were shared in an automatic Facebook update.
In the final group, the system updated walkers' Facebook feeds with both the goal and the outcome (in addition to Facebook sharing, for the last two groups the programme sent emails to select groups of supporters that each participant chose).
Walkers made fewer commitments when they knew their goals would be made public. In the private group, participants committed 88 per cent of the weeks, compared with 78 per cent of weeks when the commitments were shared and 77 per cent when both the commitment and the outcome was made public.
While it appears that public accountability decreased goal setting, it also had some counteracting positive effects, the researchers said.
One was that walkers whose commitments were posted publicly were buoyed by supportive friends and colleagues.