This talk will cover our approach to testing of (and with) cyber-cyber digital twins. A cyber-cyber digital twin is a deployed software model that executes in tandem with the system it simulates, contributing to, and drawing from, that system’s behaviour. This talk outlines Meta’s cyber-cyber digital twin, WW, a twin of the Meta WWW platforms, including Facebook and Instagram. WW is built using Web-Enabled Simulation. The talk will cover research challenges and opportunities in the general area of software testing and will include opportunities and open problems for further research on metamorphic testing. Metamorphic testing is well-suited to the problem of testing of (and with) cyber-cyber digital twins, because these twins involve inherently unknowable test oracles, thereby denoting the most pernicious form of oracle challenge for automated software testing. This talk reports the results of joint work with Sophia Drossopoulou, Inna Dvortsova, Natalija Gucevska, Michal Krolikowski, Maria Lomeli, Simon Lucas, Rubmary Rojas, and Silvia Sapora.