We study the effect of social information on the voluntary provision of public goods. Competing theories predict that others' contributions might be either substitutes or complements to one's own. We demonstrate a positive social information effect on individual contributions, supporting theories of complementarities. We find the most influential level of social information is drawn from the 90th to 95th percentile of previous contributions. We furthermore find the effect to be significant for new members but not for renewing members. In the most effective condition, social information increases contributions by 12% ($13). These increased contributions do not crowd out future contributions.