Social media are now an important aspect of the professional lives of school teachers. This paper explores the growing use of mass 'teacher groups' and 'teacher communities' on social media platforms such as Facebook. While these online communities are often welcomed as a means of professional learning and support, the paper considers the extent to which Facebook groups also expose teachers to some of the less beneficial aspects of social media, such as various forms of 'digital labour', commercialisation of exchanges and predominance of individualised reputation-driven behaviours. Drawing on a detailed examination of a Swedish teacher Facebook group of over 13,000 members, the paper first addresses aspects of the online community that could be seen as professionally beneficial and/or valuable--particularly in terms of information exchange and social support. Yet while perceived by participants as a relatively beneficial and uncontroversial aspect of their working lives, the research also points to characteristics of the Facebook group that constituted disadvantaging, exploitative and/or disempowering forms of technological engagement. In these terms, the paper highlights tensions between what appears to 'work' for individual teachers in the short term and likely longer-term implications that these practices might have for diminished professionalism and expertise of teachers.