The crowdsourcing business model has gained increasing popularity over the last years. When requesters (companies, institutions or individuals) crowdsource tasks, they place them on a platform where contributors can view and complete them. This study assesses what factors motivate contributors to complete crowdsourcing tasks and whether and to what degree these factors differ across crowdsourcing platforms. This study compares the results of quantitative surveys of contributors on two crowdsourcing platforms: RapidWorkers and Amazon Mechanical Turk. The results show that contributors on both platforms are motivated by payment and by indirect feedback from the job. However, contributors on RapidWorkers are also motivated by enjoyment-based factors, whereas contributors at Mechanical Turk are also motivated by community-based factors and delayed payoffs. Thus, the contributors at RapidWorkers tend to be more extrinsically motivated, while the contributors at Mechanical Turk tend to be more intrinsically motivated.