An underdeveloped theme in scholars' understanding of the personal services sector of the platform economy-also known as the "sharing economy"-is change. Most research on ride-hail, food delivery, accommodations, and other personal services has offered largely essentialist accounts. In this paper, we focus on how platforms have become increasingly "commercialized." In labor-intensive services, commercialization occurs as a growing fraction of the work is done by a core of full-time, dedicated workers. However, platforms that rely primarily on capital may display similar dynamics, in which a small number of participants account for the majority of activity and capture the largest share of value. In this paper, we present the first comprehensive account of commercialization of a major platform. We analyze how Airbnb markets in the 10 largest short-term rental markets in the United States changed between 2015 and 2019. We find considerable evidence of commercialization, as a rising majority of properties are rented on a very frequent basis, and casual listings, while still present, are a small and falling percentage. Relying on an original database of regulations, we show that enactment of even the strictest regulations has not durably reduced the number of listings and has had limited success in altering the mix of commercialized and casual listings over this period. We also consider the impact of COVID-19 on this platform and the sharing sector. We conclude that the short-term rental market on Airbnb has become a fairly conventional one, with little of the peer-to-peer character of its earlier days.