For millions around the globe, digital payment apps such as Venmo are replacing cash as the preferred method of payment between friends and vendors. Apps like Venmo bring a unique blend of convenience and social interactions into financial transactions. In this paper, we study the role of social relationships in the adoption of the Venmo digital payment system. We collect records of all 91 million public transactions conducted on Venmo since its introduction, a social graph connecting most of its 10.5 million users, and analyze the interplay between social relationships and financial transactions. We find that Venmo communities are very densely connected compared to other interaction networks, and are often driven by specific niche applications. We are able to extract both user-to-user and user-to-vendor transaction communities, and show that they exhibit dramatically different structural properties.