Despite the rising importance and economy-wide effects of online platforms, the paucity of cross-country comparable data still hampers understanding of the structural and policy determinants of their diffusion. This study contributes to the understanding of multi-sided online platforms in three main ways. First, we build a harmonised international dataset of online platforms and their use across 43 OECD and G20 countries, covering the 2013-19 period and nine areas of activity. Second, we describe main trends in the use of platforms in the past years, and third, we investigate the structural and policy determinants of online platforms diffusion across countries and over time.