National social movement organizations (SMOs) with widely dispersed local affiliates have been common since the turn of the twentieth century when prototypical groups such as the National Association for the Advancement of Colored People (NAACP) were founded. Social movement groups like them have continued to proliferate up to the present. At the turn of the twenty-first century, I will show, approximately one quarter of local SMOs in the United States are affiliates of national ones. The majority of those local affiliates work to mobilize members and adherents for collective action projects. This portrait of the U.S. social movement sector, however, is at odds with increasingly widespread understandings of the predominant modern social movement form – “advocates without members” (Putnam 2000 and Skocpol 1999a) – and images of typical recent citizen behavior – “civic disengagement.” The several intersecting puzzles that emerge from these contrasting images motivate the analyses that follow.