As teachers' informal professional development is visible in social media, this study probes teachers' participation in self-organized Facebook groups in mathematics or Swedish-language education. In total, 553 posts from six Facebook groups were categorized using Shulman's knowledge-base framework, and analysed using systemic functional grammar. Teachers use "questions" and "offers" most frequently (88%). Within these speech functions, pedagogical content knowledge dominates (63%), indicating that these groups constitute professional learning communities that teachers use as a professional development resource, focusing the interaction on pedagogical content knowledge. This study finds a largely similar practice in Facebook groups across the two subjects.