We study the market for apps on Facebook, the dominant social networking platform, and make use of a rule change by Facebook by which highly engaging apps were rewarded with further opportunities to engage users. The rule change led to new applications with significantly higher user ratings being developed. Moreover, user ratings became more important drivers of app success. Other drivers of app success are also affected by the rule change; sheer network size became a less important driver for app success, update frequency benefitted apps more in staying successful, and active users of Facebook apps declined less rapidly with age. Our results show that social media channels do not necessarily have to be managed through hard exclusion of participants but can also be steered through "softer" changes in reward and incentive systems.