How important are social relationships for contribution to collective action? Existing work on contribution to collective action and participation in social movements has shifted its focus from individual characteristics to social structure as the key to predicting participation, but much of the work on social structure focuses on dyadic interactions as predictors of initial contribution. Building on recent research on patterns of group joining behavior, both online and in academic research settings, this study explores the extent to which attributes of dyadic and local network structure predict continued participation. The research presented here uses data from Wikipedia, the online encyclopedia, and Wallop, a social networking and personal publishing service, to explore four key questions. First, how do dyadic relationships affect rates of continued participation? Second, what is the relationship between local network structure, such as triadic closure, and subsequent contribution? Third, if triadic closure has an effect, is it the result of structural differences in the network, or is triadic closure encoding dyadic tie strength? Finally, are social network attributes predictive of contribution in task-oriented groups, or are they more important for socially oriented groups? The results of this study highlight the importance of social relationships with other contributors as predictors of participation in voluntary associations. The importance of local network structure, which does not appear to be an artifact of tie strength, suggests a possible social affirmation effect where individuals are motivated to contribute by relationships embedded within a social group of other active participants. Furthermore, while social interaction is predictive of increased participation in both socially oriented and task-oriented systems, strong social relationships are negatively correlated with contribution in task-oriented settings. This suggests competing effects, where social relationships with other contributors serve as an important attraction even as they distract the contributor from the task at hand.