The concept of social capital has attracted much attention from researchers and policy makers, largely due to the wealth of evidence linking it with a variety of positive social outcomes, including philanthropic acts such as volunteering and donations. However, the rapid growth in Internet technologies and social media networks have fundamentally affected the formation of social capital, as well as the way in which it potentially associates with prosocial behaviors. This study uses unique data from a survey of online volunteers to explore the interrelationships between social capital and philanthropic activity in both online and offline settings. Our results show that while social capital levels associate strongly with offline donations, there are key differences in the relationships between social capital and volunteering in online and offline settings. Using novel instruments as part of a 2SLS regression analysis, we also infer a number of causal relationships between social capital and philanthropy.