The aim of this paper is twofold: (1) to conceptually understand membership dynamics in the open source software (OSS) community, and (2) to explore how different network characteristics (i.e., network size and connectivity) influence the stability of an OSS network. Through the lens of Ising theory, which is widely accepted in physics, we investigate basic patterns of interaction and present fresh conceptual insight into dynamic and reciprocal relations among OSS community members. We also perform computer simulations based on empirical data collected from two actual OSS communities. Key findings include: (1) membership herding is highly present when external influences (e.g., the availability of other OSS projects) are weak, but decreases significantly when external influences increase, (2) propensity for membership herding is most likely to be seen in a large network with random connectivity, and (3) for large networks, when external influences are weak, random connectivity will result in higher network strength than scale-free connectivity (as external influences increase, however, the reverse phenomenon is observed). In addition, scale-free connectivity appears to be less volatile than random connectivity in response to an increase in the strength of external influences. We conclude with several implications that may be of significance to OSS stakeholders in particular, and to a broader range of online communities in general.