As audience capacity is a fundamental index in ads campaigns, managers of online advertising communities always try to attract as many audiences as possible. However, due to the limited attention, effectiveness (click-through-rate) of advertisements is negatively influenced by the huge amount of information created by audiences in communities. At the same time, the great number of audiences in communities can generate greater social influence that may also affect the click-through-rate. Thus, it is necessary to figure out the optimal size of online communities. In this paper, we employ and model the effect of information congestion and social influence to solve the problem. According to the model, we find that more audiences do not mean more profit. At last, we point out some potential directions to improve the research in future, including proposing more realistic functions, relaxing control of posting position and time, and incorporating pricing strategy and communities’ competitions.