To date, little empirical research on professional use of social network sites has been conducted, particularly with reference to groups of teachers on Facebook. The paper presents the results of two surveys addressed to the founders of five Italian Facebook groups and their members ( n = 1107), with the aim of investigating mechanisms underlying group membership and reflecting on their implications for professional development. A number of hypotheses were tested in order to explore the nature of three dimensions (domain, network and practice) involved in these groups, assimilated by authors into networks of practice. The study investigated the mechanisms of group membership and their participatory dynamics in terms of group types (generic vs. thematic), duration of membership and the interplay between offline and online activities. Results showed that groups differing in terms of domain (generic or thematic) are characterized by particular mechanisms of affiliation and participation and that mechanisms of legitimation of the members, as well as of shared resources, are associated with the duration of membership. Further research should be conducted to investigate how social capital dynamics evolve over time, influencing the construction of group identity. Practitioner Notes What is already known about this topic The use of social network sites ( SNS), particularly Facebook, is growing in the educational field., To date, little attention has been given to the use of Facebook for professional development., Empirical research mainly refers to the higher education context and shows that the use of SNS has positive outcomes for social capital., What this paper adds An application of social capital theory with the networks of practice framework (Brown & Duguid, ) to the study of professional groups of teachers in SNS., An understanding of groups in terms of shared social capital, both bridging and bonding., An analysis of the interplay between online and offline activities in terms of professional benefits for teachers., Implications for practice and/or policy Participation in this kind of groups may have a positive impact on professional development., Social networks may afford different types of social capital for professionals., Policy makers should encourage and support the use of social networks for professional learning. [ABSTRACT FROM AUTHOR]