This study aims to reveal the mechanism of how consumers’ engagement is established in social commerce communities. Building upon social support theory, we propose models and employ datasets obtained from Douban.com to empirically investigate the different effects of emotional and informational support on consumers’ engagement and how involvement mediates these relationships in the social commerce context. At the same time, the moderating role of product presentation is examined to help clarify the impact of social support and consumer involvement. Our analyses reveal that the two subdimensions of social supportive information are positively related to consumer involvement, together promoting consumers’ engagement in the community. In particular, involvement imposes a full mediating influence on emotional support but a partial mediating impact on informational support. Product presentation strengthens the effects in that the predictive effect of determinants on engagement is more pronounced for books with electronic versions in the social commerce community. For researchers and managers, these findings emphasize the benefits of the social commerce community, particularly with regard to Douban.com in China.