Charitable giving and social group size: Does it take a lot of friends to be a successful fundraiser?

Much charitable giving takes place in the context of existing social groups (workplaces, churches, friends and family). We exploit a real-world setting (online fundraising) to provide the first evidence on how the size of the social group affects donations. We show that the number of Facebook friends of individual charity fundraisers has a negative effect on average donation size. This is consistent with free-riding behaviour only if donors care about the total amount raised by the fundraiser. We discuss other possible explanations.