The online community knowledge market has been rapidly gathering popularity in recent years. People can easily access and share information and expertise through the Web without time and geographic constraints. For example, Yahoo! Answers, launched in December 2005, has obtained 80 million unique users worldwide and approximately 23 million resolved questions.1 Similarly, in China, the biggest Internet portal, Baidu.com (the most used site in China according to Alexa.com), also provides an online community based question-answer (QA) platform. Since it began in June 2006, over 25 million questions have been answered on Baidu. 2 There are many points in the design space of these knowledge sharing communities (Wenger 1998; Ackerman et al. 2003). Yahoo! Answers allows users to ask questions about a variety of topics, and anyone can answer (Adamic et al. 2008). Google Answers, on the other hand, had vetted experts who would answer a user's question for a fee