This paper investigates the characteristics and behaviour of learners on nine Massive Open Online Courses (MOOCs) on the FutureLearn platform in 2014 and 2015. FutureLearn's social constructivist approach, which emphasizes learning through social interaction, makes the focus on forum posting within these MOOCs particularly appropriate. This study makes a significant contribution to the research on MOOCs by exploring first the extent to which learners on FutureLearn MOOCs make comments, second, whether groups of learners who comment to a lesser or greater extent have different demographic characteristics and third, whether commenting activity is associated with MOOC completion. The study uses a large dataset of over 25 000 learners to examine learners who comment versus those who do not, and learners who generate many posts. The results show that learners who are older, work part‐time or not at all are more likely to post a comment, and those who are older, work part‐time or not at all, are better educated and have prior online experience make the most comments. Making comments is also strongly associated with completing a MOOC, in particular for those who make many comments. Lay Description What is already known about this subject matter? MOOCs have low completion rates. There are different types of MOOCs with different pedagogical approaches. FutureLearn MOOCs have a social constructivist approach and provide discussion areas to encourage social interaction. MOOC learners have different commenting behaviours. What this paper adds? A consideration of the extent to which the FutureLearn approach is successful in its encouragement of social interaction. A better understanding of which learners make comments. A large‐scale study of the relationship between commenting and completion of MOOCs. Implications for practitioners Commenting and completing seem to be related. Older, educated learners who have done an online course previously, who have spare time tend to comment and complete more. A recognition that some types of learners engage with MOOCs in a less socially interactive way.