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. (Contains 7 tables.)