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1. Introduction
In the era of Web 2.0, many types of content are generated or rated by users (Gazan, 2011; Paroutis and Al Saleh, 2009). Social question-and-answer (Q&A) sites, such as Yahoo! Answers (http://answers.yahoo.com/) and Quora (www.quora.com), which allow users to freely ask and answer questions, are typical examples of this phenomenon (Fang and Zhang, 2019; Guan et al., 2018; Jin et al., 2013, 2015; Kang, 2018; Khansa et al., 2015; Zhou, 2018).
On social Q&A sites, there are users who actively answer questions from other users. Their main activity is answering, and their asking is rare compared to answering (Kang et al., 2011). These active answerers are defined as “active users” in this study. Active users provide a core knowledge base that attracts general knowledge-seeking users to such sites (Kang, 2018). However, it is not natural to voluntarily provide knowledge to social Q&A sites. Because knowledge on social Q&A sites is a kind of “public goods” (Cabrera and Cabrera, 2002), users tend to free-ride – get answers to their own questions and do not respond to the questions from other users (Guan et al., 2018). Therefore, active users are a small portion of the entire user group, and most users are askers or readers (Kang et al., 2011). They will stop visiting if they fail to find the answers they need. Even among answerers, many of them stop answering after trying one or two answers (Pudipeddi et al., 2014). Thus, active users who repeatedly share knowledge are valuable assets of social Q&A sites, and maintaining their knowledge sharing behavior is crucial for the sustained success of such sites (Fang and Zhang, 2019).
As a practical way to maintain active users’ contributions, social Q&A sites usually have special programs to identify and motivate active users. For example, Naver Knowledge-iN awards top-ranking active answerers with the title “Power Knowledge-iN (KiN).” Similarly, Quora selects “Top Writers” every year, and Yahoo! Answers has a “Leaderboard” to identify the most active users. However, these efforts of social Q&A sites identifying active users and appreciating their contribution officially are not enough to ensure active users’ continuous knowledge sharing. Among many social Q&A sites with similar functions, only a few are operating successfully (Guan