The Canadian Refugees Welcome Movement (2015-2016) was one of the most sizeable, visible, and effective instances of collective action in recent Canadian history. It had a nationwide scope and grassroots initiation. It comprised a wide variety of participants and actively employed social media in its constitution. This article reports the results of a multimethod case study that seeks to explain how collective action frames emerged in the context of the Canadian Refugees Welcome Movement; which actors were involved in their articulation; and how they generated a following, collective action and humanitarian and political effect. The focus is on the discursive processes of construction of solidarity across difference as they unfolded in the social media environment. The Facebook event pages calling for rallies in support of Syrian refugees, it argues, served as a discursive space that helped transform the moral shock experienced by members of distinct moral communities into a process of concerting of voices and construction of solidarity and collective action frames across differences. Résumé Le mouvement canadienne d'accueil des réfugiés (2015-2016) a été l'un des exemples d'action collective les plus importants, visibles et efficaces de l'histoire récente du Canada. Il avait une portée nationale et une initiation à la base. Il comprenait une grande variété de participants et utilisait activement les médias sociaux dans sa constitution. Cet article présente les résultats d'une étude de cas multi-méthodes qui cherche à The research presented in this article was supported by SSHRC Insight Grant 435-2014-0200.