Electronic cigarette (e-cigarette) Internet forums are social networking websites that allow members around the world to share information and personal experiences about e-cigarettes. However, little is known about their popularity and reach. This longitudinal study investigated indicators of participation (membership, number of discussion boards, and number of message posts) in 77 e-cigarette forums from February 2016 to January 2017. Autoregressive time-series analyses were used to assess monthly changes in participation among small (5000 members) forums that were active at all 12 observation time points. Paired sample t-tests were conducted to compare changes in participation before and after the U.S. Food and Drug Administration announced the regulation of e-cigarettes. Analyses revealed significant and consistent increases in forum membership (R2s > 0.72; ps  0.97; ps  0.96; ps < 0.001) across all forum sizes during the 12-month period. No differences were found in participation indicators pre- and post-regulation announcement. E-cigarette forums provide real-time information on the levels of participation over time and therefore may be an important platform to study interactions among consumers and how these interactions shape e-cigarette knowledge, attitudes and use behaviors.