Asynchronous online discussions are broadly used to support social learning. This paper reports on an undergraduate class's online discussion activities over one semester. Applying social network analysis, this study revealed a participation gap among students reflected by their varied levels of network prestige. The low-prestige group initiated equivalent volumes of interactions but were less reciprocated. In-depth analysis found the high-prestige group also advantageous in other network measures such as closeness centrality and eigenvector centrality, as well as the strength, persistence, and reciprocity of their ties. To probe potential explanations of the revealed gap, we further contrasted post content and posting behaviours between two groups. Results did not identify any significant differences in post content but found low-prestige students' participation less timely and more temporally compressed. This paper calls for attention to the participation gap in online discussions, microlevel temporal patterns of student activities, and practical means to scaffold student participation in asynchronous online discussions.