How does the rise of managed online platforms for civic engagement change the relationships between activists and organizations? While much has been written about Twitter- and Facebook-enabled mobilization, the emergence of platforms that organize “microaction” in contained ways is a phenomenon understudied by social movement and media scholars. This study draws on both literatures to analyze the hybrid case of Sparked, a microvolunteering platform created by web designers, not activists, that efficiently organizes volunteering through a microaction design. The case exhibits characteristics that social movement scholars understand as resource mobilization through leveraging of affordances by activists, but it also features the structural characteristics of platforms that media scholars identify as both enabling and constraining. To conceptualize the “digitally enabled activism” that takes place within the confines of a managed platform, this study investigates its microaction affordances and their implications. My analysis finds that high leveraging of online affordances can coincide with a shifted logic of engagement in the case of microaction platforms: Sparked’s microaction system affords high performance, but targets only a specialized niche of volunteering. I describe this model as “specialized supersizing” that addresses nonprofits’ needs for increasing organizational overhead services in the context of a professionalizing third sector. Sparked’s microvolunteering design has helped define a platform-centric constitution for microaction that is geared toward rationalization, professionalization, and productivity. Its platform “politics” promote depoliticization, where tactics, not causes, determine exchanges. This emergence of a market for mobilization tactics may have important implications for nonprofits, volunteers, and social movements.