With a growing number of teachers engaging online with their peers, online social spaces are increasingly highlighted as playing a key role in teachers' professional learning and development. However, while academic and professional discourses tend to focus on the benefits and weaknesses of teachers' engagement in online social spaces, little attention has been given to the spaces themselves. Rooted in essentialist or instrumentalist assumptions about technology, these spaces are often conceptualised as neutral contexts, free from values, structures, and agendas that simply facilitate interaction. However, presenting data gathered during a year-long digital ethnography of three such spaces, this paper argues that they are in fact highly complex social environments that contain embedded structures relating to technical design and functionality, dominant discourses, and the agendas of parent organisations. Such structures shape user engagement and have a constructive influence over the ways in which teachers think about their subjects and themselves as professionals. Therefore, it is argued that in order to understand fully the place online social spaces could or should have in teachers' professional lives, the complex online environments in which teachers engage and the relationship between structure and agency must be analysed.