New approaches are needed to measure innovation in peer communities. Based on interviews with members of three online communities, we investigate how these networks contribute to sustainable innovation. To identify relevant interaction processes, we use the functions approach from the TIS literature. We assess the communities' contributions to system functions and identify reinforcing dynamics. Patterns of peer coordination are described that stabilise technological pathways through network effects and direct collective efforts towards options that benefit many actors in the peer community. Using metrics from the field of open and user-driven innovation, we develop a measurement framework for semi-automatic, quantitative assessment of these interactions.