This study investigated the use of Facebook as a medium for, and process of, teachers’ informal professional learning regarding mathematical and pedagogical knowledge. Facebook responses to four inter-related posts about mathematics examples from Facebook users (N = 117) were captured and systematically analysed using content analysis. The data were examined through the lens of the five characteristics of the Effective Professional Development framework. The results identified different types of responses to the four Facebook posts including the opening of learning opportunities, rich mathematical and pedagogical discussions, respondents’ appreciations and direct impacts of the posts, as well as the acknowledgement of the misalignment between the proposed ideas and current teaching practices in Indonesia. These responses provided evidence that all five characteristics of the framework were apparent and highlighted the role of Facebook in supporting teacher informal professional learning within the Indonesian context. The limitations of such social networking-based research and implications for teacher professional development are provided.