Liljekvist, Yvonne Elisabeth, et al. "Facebook for professional development: Pedagogical content knowledge in the centre of teachers’ online communities." Scandinavian Journal of Educational Research 65.5 (2021): 723-735.

This study examines teachers' participation in self-organized Facebook groups for mathematics and Swedish-language education, analyzing 553 posts from six groups using Shulman’s knowledge-base framework and systemic functional grammar. The findings reveal that 88% of the interactions are questions and offers, with a 63% dominance of pedagogical content knowledge, indicating these groups serve as professional learning communities for teacher development. The practices across mathematics and Swedish-language groups are found to be largely similar.