Social media represent an increasingly important vehicle for informal professional development amongst teachers, and provide an illuminating means of tracing their collective knowledge building. The study reported here examines six large Facebook groups, created by and for teachers in Sweden, to exchange information about the teaching of mathematics and Swedish. Analysis establishes that professional knowledge was made available in 86% of discussion threads, most commonly relating to Shulman's categories of Knowledge of Learners, Curricular Knowledge, and Pedagogical Content Knowledge. Participants acknowledged opportunities for professional learning in 88% of such threads, and showed new understanding in 11%, particularly in longer threads.