Abstract: Market thickness is a key parameter that can make or break a platform’s business model. Thicker markets can offer more opportunities for participants to meet and higher chances that a potential match would exist. However, they can also be vulnerable to potential search frictions. In this paper, using data from an online peer-to-peer holiday rental platform, we aim to identify and measure the causal impact of market thickness on matching efficiencies. In particular, we exploit an exogenous shock to market size caused by a one-time migration of listings from other platforms, which gave rise to a quasi-experimental design.