Massive Open Online Courses (MOOCs) have the potential to make quality education affordable and available to the masses and reduce the gap between the most privileged and the most disadvantaged learners worldwide. However, this potential is overshadowed by low completion rates, often below 15%. Due to the high level of autonomy that is required when learning with a MOOC, literature identifies limited self-regulated learning skills as one of the causes that lead to early dropouts in MOOCs. Moreover, existing tools designed to aid learners in the online learning environment fail to provide the support needed for the development of such skills. The aim of the present work is to bridge this gap by investigating how self-regulated learning skills can be enhanced by encouraging metacognition and reflection in MOOC learners by means of social comparison. To this end, following an iterative process, we have developed the Learning Tracker, an interactive widget which allows learners to visualise their learning behaviour and compare it to that of previous graduates of the same MOOC. Each iteration was extensively evaluated in live TU Delft MOOCs running on the edX platform while engaging over 20.000 MOOC learners over the whole duration of each MOOC. Our results show that learners that have access to the Learning Tracker are more likely to graduate the MOOC. Moreover, we have observed that the widget has a positive impact on learners' engagement and reduces procrastination. However, we have little evidence that learners improved their self-regulated learning skills by the end of the MOOCs. Based on our results, we argue that the mere fact of receiving feedback on a limited number of learning habits could trigger self-reflection in learners and lead to improved learner performance. This work underlines the powerful effect feedback and self-reflection on one's behaviour has on learning performance. We recommend that future research should investigate learners' feedback literacy and devise effective ways of presenting learners with personalised feedback based on their goals, learning skill level and cultural background.