Differences in learning gains between interactive engagement and lecture instructional practices have been well documented and yet the ways in which students participate in each of these learning environments are not clearly established. We use social network analysis as one way to establish differences the participation of students in lecture sections and students in Modeling Instruction, a curriculum that uses interactive engagement. One primary difference in the way students participate in the two instructional practices is that students in Modeling Instruction classes form learning communities and students in lecture classes remain isolated. Students in Modeling Instruction sections report ten times greater numbers of ties between students than those in lecture sections, forming richer and more deeply connected networks. We interpret these differences in terms of a participationist view on learning and as an explanatory mechanism for understanding documented differences in learning gains in the two settings. © 2010 American Institute of Physics.