Online communities play an important role in society. In this paper, we study the effects of membership overlap on the survival of online communities. By analyzing the historical data of 5673 Wikia communities, we find that higher levels of membership overlap are positively associated with greater survival rate of online communities relative to lower levels of membership overlap. Furthermore, we find that it is beneficial for new communities to have shared members who play a central role in other mature communities. These findings provide new insight into an important mechanism underlying successful online communities, contribute to theories of organization science, and provide several actionable steps for the hosts and creators of online communities.