Online communities and social-network sites are used to deliver professional-development services for teachers. Professional development should help teachers to reflect on their practice and improve in helping them to guide students' growth. Peer and community models, such as coaching and sharing knowledge in network and knowledge communities, have been proposed. Recently these practices have been taken into use in social media services, such as Facebook. Although earlier research has examined teachers' online communities, we move beyond understanding individuals motivations and examine community-level dynamics. Understanding these dynamics is important to understand the interaction between teachers, resources and the platform in use and resulting professional development. To understand the evolution of an informal and self-organised Facebook teachers' group, containing nearly 20,000 teachers, its eight years of activity was analysed by employing a mixed-methods research design; data science and participatory observation. Analysis gives account of both the evolution of participants' engagement and activity, and the evolution of content and its relevance for teachers’ professional development. The results suggest that managers of professional development need to consider how to facilitate participation in order to focus on pedagogically motivated use of information technology, for system developers to consider how to assist recruitment of members and sustain their activity, and for all stakeholders to acknowledge that a peer-organised online professional development community requires significant effort. Furthermore, we suggest that instead of addressing large groups like these as communities, scholars and practitioners should instead see them as personal learning networks and think about how to establish smaller and more manageable groups as communities.