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Some user-contributed content (UCC) applications, such as Yahoo! Answers, Wikipedia, and YouTube have drawn much media attention. Various reasons motivate the tremendous contributions to a few UCC systems so far. Existing literature has uncovered many factors that affect users’ decisions to become contributors [Bryant et al., 2005], to continue contributing [Nov, 2007], and to increase contribution [Chen et al., 2007], but none of them pay attention to why active contributors decide to stop contributing. The exit of active contributors may affect quantity and quality of content provision on UCC systems. Indeed, preliminary evidence shows that there is a reason to worry about the long term sustainability of some systems. In 2000, Adar and Huberman’s study on the Gnutella network showed that there was high level of free-riding on this network. Five years later, Hughes et al.[2005] find on the same network free-riding has gotten worse: the percentage of users who do not share any files has increased from 66% to 85%. In the mean time, Andrew Lih, one active Wikipedia research/contributor also blogged about its growth rate slowing down dramatically in late 2006 [Lih, 2007]. To what extent is this due to contributors leaving the system? If so, who left? And for what reasons? Answers to these questions are useful to UCC system designers for determining the impact of contributors leaving or for devising mechanisms to prevent them from leaving. In this study we focus on one system, Wikipedia, and analyze why some editors stop contributing. We chose Wikipedia for a number of reasons. First, it has a large number of active contributors. By mid …