Chatbots are computer programs aiming to replicate human conversational abilities through voice exchanges, textual dialogues, or both. They are becoming increasingly pervasive in many domains like customer support, e-coaching or entertainment. Yet, there is no standardised way of measuring the quality of such virtual agents. Instead, multiple individuals and groups have established their own standards either specifically for their chatbot project or have taken some inspiration from other groups. In this paper, we make a review of current techniques and trends in chatbot evaluation. We examine chatbot evaluation methodologies and assess them according to the ISO 9214 concepts of usability: Effectiveness, Efficiency and Satisfaction. We then analyse the methods used in the literature from 2016 to 2020 and compare their results. We identify a clear trend towards evaluating the efficiency of chatbots in many recent papers, which we link to the growing popularity of task-based chatbots that are currently being deployed in many business contexts.