Professional development programs provide an opportunity for environmental educators to develop networks to exchange ideas and practices in professional learning communities. This study investigated how diverse educators develop professional networks for exchanging information through online and face-to-face professional development activities. We conducted surveys and social network analysis of three professional learning communities including a state consortium with mostly face-to-face activities, an online-only learning community, and a fellowship program with both face-to-face activities and online interaction. We then used exponential random graph models to examine factors that influenced professional network formation in each community. The results showed that professional development activities including face-to-face meetings and Facebook interactions, and collaborative projects such as co-authoring book chapters, had positive effects on the likelihood of network formation. Results suggest that program leaders can facilitate discussions and collaborative projects to foster network formation in professional development programs, especially those conducted exclusively online.