Our study investigates the use of a new open source platform in catalyzing social innovations and participation of its members over time. We empirically examined how the nature of project designs and social pressure affect contribution to the open source platform. In the twenty-one projects (3, 998 contributions) from 2004 to 2009, we find that the average number of contributions is higher when the projects are highly visible, when the project is designed to require specific skills from participants, and when it requires outcome measurement from participants’ proposals. Also, we verified that actors join collective action when they believe their contribution is meaningful and they would stop when they believe their contribution could be marginal. These results provide implications for open source platform design in the philanthropic sector.