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.

在线社区和社交网站用于为教师提供专业发展服务。专业发展应该帮助教师反思自己的实践，并在帮助教师指导学生成长方面进行改进。已经提出了对等和社区模型，例如在网络和知识社区中指导和共享知识。最近，这些做法已被用于社交媒体服务，例如 Facebook。尽管早期的研究考察了教师的在线社区，但我们超越了理解个人动机和考察社区层面的动态。了解这些动态对于了解教师、资源和所用平台之间的相互作用以及由此产生的专业发展非常重要。为了了解一个包含近 20,000 名教师的非正式和自组织的 Facebook 教师小组的演变，通过采用混合方法研究设计对其八年的活动进行了分析；数据科学和参与式观察。分析考虑了参与者参与和活动的演变，以及内容的演变及其与教师专业发展的相关性。结果表明，专业发展的管理者需要考虑如何促进参与以专注于信息技术的教学动机使用，让系统开发人员考虑如何协助招募成员并维持他们的活动，并让所有利益相关者承认同行组织的在线专业发展社区需要付出巨大的努力。