Social media, not least Facebook, has in recent decades emerged as an important arena for interaction and identity formation. The overall aim of the thesis is to contribute with knowledge about how collective identity formation takes shape within groups in social media where social issues are discussed. Specifically, collective identity formation has been studied in three Facebook groups with different orientations. The theoretical starting point is constituted by a combination of a socio-cultural perspective and Goffman’s dramaturgical perspective. Methodologically, a qualitative, netnographic approach is used, supplemented with qualitative content analysis and conversation analysis. The results show how interaction in the Facebook groups is enabled and limited by the platform’s technical conditions as well as by different social conditions, norms and rules that frame these practices. Three types of interaction are distinguished, they are characterized by resistance, agreement and nuance respectively. Based on these three types of interactions, three collective identities are presented: Resistance identity, is formed through interaction that manifests a shared distance from something/someone; Solidarity identity, is formed through interaction that manifests common advocacy and/or celebration of something/someone and Negotiation identity, is formed through a joint effort to reflect on, negotiate and nuance the understanding of something/someone. From this overall result, two aspects are highlighted in the concluding discussion. The first aspect concerns interaction in social media and the second aspect concerns collective identity formation as learning.