Fentanyl sale is often promoted on social media, but there is very little empirical and systematic research on this issue. This study attempts to fill a gap in the literature by utilizing big data and computational as well as qualitative analyses to better understand this phenomenon. The study’s sample includes a unique dataset of over 6 million tweets and Instagram posts referencing fentanyl including 5687 messages (856 tweets and 4831 Instagram posts) that promoted fentanyl sale and provided some instructions on where and how to purchase it. From a theoretical point of view, it discusses the concept of Dark Social Networking Sites (DSNS) by building and expanding on the previous literature on dark social media. Second, it offers an assessment of the content posted and major strategies used by opioid drug dealers on social media which mostly involve exchanging contact information of mobile communication apps in order to arrange for money transfers and deliveries.