Despite the growing academic literature on the World Social Forum process, few scholars have attempted to systematically analyze the social, cultural, and political impact of the forums. This has to do in part with the inherent difficulties of assessing movement consequences, which is particularly complicated for an activity geared toward creating ‘open spaces.’ This article presents an analytic framework for evaluating the impact of the social forums through an analysis of the 2010 United States Social Forum (USSF) in Detroit from the perspective of a local Boston-based delegation called the Boston Freedom Rides. We then use that framework to consider the impact of the 2010 USSF, bridging the academic literature on movement outcomes with activist perspectives. We make two related claims. First, the social forums, and the USSF in particular, should be viewed and their impact assessed in light of their generativity as ‘movement-building machines’: infrastructures designed for the production of social capital, networks, solidarities, meanings, frames, identities, knowledges, strategies, skills, and repertoires. Second, with respect to the Freedom Rides, the 2010 USSF contributed to movement building on multiple levels, but more so within rather than across movement sectors. Our goal is less to make a definitive argument about the impact of the 2010 USSF than to provide a helpful way of thinking about movement building as a social movement outcome, which can be applied and refined through further comparative and longitudinal research. We thus favor breadth over depth in outlining a broad framework for future inquiry.