The purpose of this study is to examine the effect of material incentives on the knowledge-sharing networks and information lifecycles of a public online forum. The users' interaction data were collected from two similar sub-forums, and one of them provided material incentives for sharing knowledge. After collecting user behaviors for one natural year, the interaction data from 18,079 main posts and 326,024 replies were coded using social network analysis. Results showed that both sub-forums have significant small-world effects, but the effect is more obvious in the incentive intervention, with higher density, larger network cohesion, shorter average path length, and larger power-law index. Also, the material incentives made the whole interactive network more connected, with less isolated clusters. According to the K-core analysis, marginal users in the incentive intervention relied less on core users and had been motivated toward the network center. However, the information lifecycle for both conditions was about 46 days, with no significant difference. These findings indicated that the material incentives have a significant spatial effect that helps widening knowledge-sharing scopes and deepening the interaction, while it has limited time-line effects on prolonging knowledge-sharing sustainability.