Recent years have witnessed an interesting phenomenon in which users come together to interrogate potentially harmful algorithmic behaviors they encounter in their everyday lives. Researchers have started to develop theoretical and empirical understandings of these user-driven audits, with a hope to harness the power of users in detecting harmful machine behaviors. However, little is known about users' participation and their division of labor in these audits, which are essential to support these collective eorts in the future. Through collecting and analyzing 17,984 tweets from four recent cases of user-driven audits, we shed light on patterns of users' participation and engagement, especially with the top contributors in each case. We also identied the various roles users' generated content played in these audits, including hypothesizing, data collection, amplication, contextualization, and escalation. We discuss implications for designing tools to support user-driven audits and users who labor to raise awareness of algorithm bias. CCS CONCEPTS• Human-centered computing ! Human computer interaction (HCI); Empirical studies in HCI .