What determines open source project success? In this study, we investigate the impact of network social capital - the benefits open source developers secure from their memberships in a developer collaboration network - on open source project success. We focus on one specific type of success as measured by the productivity of open source project team. Specific hypotheses are developed and tested on a longitudinal panel of 2378 projects hosted at Sourceforge. We find that network social capital is not equally accessible to or appropriated by all projects. Our main results are (1) teams with greater internal cohesion are more successful, (2) external cohesion (cohesion among the external contacts of a team) has an inverse U-shaped relationship with the project's success; moderate levels of external cohesion are the best for a project's success, rather than very low or very high levels of this variable, (3) the technological diversity of a contact also has the greatest benefit when it is neither too low nor too high, and (4) the number of direct and indirect external contacts are positively correlated with a project's success with the effect of the number of direct contacts being moderated by the number of indirect contacts. These results are robust to a number of control variables and alternate model specifications. Several theoretical and managerial implications are provided.