As online communities, such as social network sites, mature, they face challenges in sustaining user engagement. To address this, we designed and deployed a rating system to encourage a broad set of users to promote a diverse set of content on a social network site. By evaluating the impact of the promotions on the site’s content and users over time, we found that the system successfully promotes more diverse content and encourages new social interactions between users. Categories and Subject Descriptors H.5.3 [Information Interfaces and Presentation]: Group and Organization Interfaces: Web-based interaction. General Terms Human Factors. Keywords Online community, social networking, ratings, rewards.