Facebook is building a digital twin of its full platform infrastructure called WW (essentially a realistic simulation, on real WWW infrastructure, that allows it to detect and prevent harmful behaviors on the real platform). WW uses many techniques, including ML, in order to train bots to simulate real user interactions. This talk presents an overview of Facebook’s WW system, its testing (using automated regression, end-to-end and metamorphic testing), and exciting open problems and applications of Software Testing for Smart Simulation systems built using ML.