This study explored parental engagement, child BMI and secondary outcomes from the social media component of an online healthy lifestyle program for parents of preschool-aged children. Intervention group participants received access to an online program and Facebook group. Data were collected at baseline and 3- and 6-months follow-up. Facebook usage data on comments and posts were used to determine total active engagement. There was a high level of Facebook group membership and most parents actively engaged at least once. Although there were varying levels of engagement between modules and cohorts, it was modest overall. User acceptability of the Facebook group was lower than expected. Children of parents in the intervention who engaged more in the Facebook group (by posting and commenting) demonstrated greater sleep duration over time (estimate 1.79, 95% CI 0.42 to 3.17, p =.01) Children of parents who engaged more in the Facebook group also participated in less moderate- to vigorous-intensity physical activity (estimate −0.14, 95% CI −0.26 to −0.01, p =.03). This study is one of the first parent-focussed healthy lifestyle interventions to include a social media component. Further research is recommended with larger sample sizes and longer duration to further explore the potential of social media in childhood obesity interventions.