Payment infrastructures are going through rapid change with the rise of next generation mobile networks and smartphone ownership. From mobile wallets to rideshare apps, social payments allow users to split receipts with friends, charge exes for breakup expenses, or troll celebrities. New layers of data, sociality, and markets are being created and influenced by expanding economic imaginaries, regulations, and business models leveraging these new infrastructures. In this paper we discuss how mobile payment systems have become social media. After discussing the recent history of mobile payments innovation---SMS, mobile wallets, delivery and ridesharing apps--- we examine Venmo, a social payments platform that allows users to broadcast transactions to a social activity stream or public transaction feed. Our findings detail how transaction feeds of mobile payments support social practices, communication, and commerce with mobile devices and wireless networks. We present findings from a case study on Venmo to develop some implications for the design, study, and impact of mobile payment infrastructures as social media.