In recent years, many people's life style has gradually become sedentary, and therefore lack of physical activity. While there are many applications that encourage physical activity through social interaction (such as rankings, sharing, or messaging), few of them have shown that community composition and trends can help people do more physical activity. Here, we created fictitious communities with different trends to investigate how different community trends affect users. We found that most of the participants in the growth community had more walking steps and all participants in the decline community had fewer walking steps than before. The fictitious community trend displayed through the leaderboards might allow our participants to experience the process of social learning and social comparisons implicitly and then change their intentions and behaviors. The results of this study provide some design implications for building fictitious communities in mobile applications to encourage users to do more physical activity.