Millions of businesses join digital platforms each year, yet little is known about their success rates or how platforms manage their entry. Using data from a leading Chinese e-commerce platform, we show that fewer than 1% of new, ID-verified sellers become successful small-to-medium-sized businesses. Analysis of search sessions reveals that consumers prefer entrant products over incumbent ones. Further, increasing entrant exposure on the margin increases overall purchase rate of a search session, indicating that entrants are under-explored compared to the social optimum. Finally, we revisit a prior intervention on the platform that increased entrant advertising. We find that it promoted entry with minimal impact on screening efficiency. A model of platform-mediated entry explains our findings and predicts how platform maturity, entrant quality, and advertising jointly determine optimal exploration.