This study examined use of an online social media physical activity (PA) challenge to encourage participants to complete at least 13 races in the year 2013. Following the challenge, researchers recruited participants through the Facebook group. Using the #13in2013 Twitter hashtag, participants completed an online survey. Survey participants reported completing a significantly greater mean total number of races during the challenge (17.5 ± 6.9 races) than the previous year (8.6 ± 7.4 races, p = .001) and 93% (n = 56) reported completing at least 13 races. Participants completed a mean of 1.4 ± 2.1 virtual races. Mean reported number of total miles ran/walked during the challenge was significantly greater (718.4 ± 465.0 miles) than the previous year (462.1 ± 436.5 miles, p = .002). Mean reported participant body weight post-challenge (68.2 ± 13.1 kg) was significantly lower than reported weight pre-challenge (69.7 ± 14.4 kg, p = .012). This study demonstrated that a social media PA challenge has the potential to present a scalable way to motivate individuals to participate in PA and decrease body weight.