•We explore the influences of perceived online community support and member relations on commitment.•Perceived online community support has different effects on commitment between posters and lurkers.•Trust positively moderates the effect of perceived freedom of expression on commitment among lurkers.•The norm of reciprocity negatively moderates the effect of perceived freedom of expression on commitment among lurkers. Based on commitment theory and organization support theory, this study explores the effects of perceived online community support on member commitment and how these effects are moderated by members’ social relations in terms of posters and lurkers. The results indicate that perceived online community support had different effects on commitment between posters and lurkers. Trust was found to positively moderate the effect of perceived freedom of expression on commitment among lurkers; on the contrary, the norm of reciprocity was found to negatively moderate the effect of perceived freedom of expression on commitment among lurkers. The theoretical and practical implications are also discussed.