Social question and answer sites (SQAs) are increasingly popular knowledge sharing platforms. In this paper, we outline how an SQA site functions as a social learning community. The success of an SQA site depends not only on effectively organizing and delivering information, but also on whether it can provide the cues needed by community members to successfully learn to be productive contributors. We explore this learning process in four different SQAs that utilize the Stack Exchange platform: Science Fiction & Fantasy, Seasoned Advice (cooking), Database Administrators, and Android Enthusiasts. Using longitudinal fixed effects models, we examine whether users learn to be better question askers over time, and how user interface features and community norms affect the cultivation of this critical skill. The study offers design implications by highlighting factors that help users develop into literate and productive community contributors in social platforms.