People often turn to their social networks to fulfill their information needs. We conducted a study of question asking and answering (Q&A) behavior on Twitter. We found that the most popular question types were rhetorical and factual. Surprisingly, along with entertainment and technology questions, people asked personal and health-related questions. The majority of questions received no response, while a handful of questions received a high number of responses. The larger the askers’ network, the more responses she received; however, posting more tweets or posting more frequently did not increase chances of receiving a response. Most often the ‘follow’ relationship between asker and answerer was one-way. We provide a rich characterization of Q&A in social information streams and discuss implications for design.