Online purposive samples have unknown biases and may not strictly be used to make inferences about wider populations, yet such inferences continue to occur. We compared the demographic and drug use characteristics of Australian ecstasy users from a probability (National Drug Strategy Household Survey, n = 726) and purposive sample (online survey conducted as part of a mixed-methods study of online drug discussion, n = 753) using nonparametric (bootstrap) and meta-analysis techniques. We found significant differences in demographics and drug use prevalence. Ideally, online purposive samples of hidden populations should be interpreted in conjunction with probability samples and ethnographic fieldwork.