Participant attrition is a significant challenge for longitudinal studies, particularly those involving disadvantaged populations. We attempted to re-engage n = 990 families in a five-year follow-up of a randomised controlled trial of an early childhood parenting intervention. Tracing was attempted for n = 90/990 (9.1%) uncontactable participants. Evidence for the identification of a matching Facebook profile was classified as strong, moderate or weak. A private message was sent to those with strong evidence (n = 63/90, 70.0%), of which 32 responded, 22 agreed to participate and 19 provided data. Compared to participants engaged using traditional methods, those re-engaged through Facebook were more likely at initial recruitment to have a younger child (p =0.02), to be a single parent (p =0.04), less educated (p <0.001), lower income (p =0.01) and in an unemployed household (p <0.001). Findings suggest that social media can be an effective engagement tool, helping to minimise overall and selective attrition in longitudinal studies, particularly for hard-to-reach populations.