1. A method implemented by a data processing apparatus, the method comprising: receiving, by the data processing apparatus, a question from an asker in which the question is associated with one or more topics; selecting, by the data processing apparatus, a plurality of candidate answerers based at least partly on each candidate answerer's respective social relationship to the asker within a computer-implemented social network; determining, by the data processing apparatus, a respective wait time for each candidate answerer in the plurality of candidate answerers, where the respective wait time is based on one or more of a respective communication channel used by the candidate answerer and a historical responsiveness of the candidate answerer for the respective communication channel, wherein the respective communication channel comprising one of an instant message, an electronic mail, a blog post, and a short message service message; selecting, by the data processing apparatus, a first candidate answerer based on a ranking of the plurality of candidate answerers; sending, by the data processing apparatus, the question to the first candidate answerer of the plurality of candidate answerers through the respective communication channel of the first candidate answerer; determining, by the data processing apparatus, that the respective wait time of the first candidate answerer for a first answer from the first candidate answerer has expired without receiving the first answer, and in response: selecting a second candidate answerer based on the ranking of the plurality of candidate answerers, and sending the question to the second candidate answerer of the plurality of candidate answerers through the respective communication channel of the second candidate answerer; receiving, by the data processing apparatus, a second answer to the question from the second candidate answerer; and sending, by the data processing apparatus, the second answer to the asker and information that identifies the second answerer.