13. A system comprising: a processor; and computer-readable medium coupled to the processor and having instructions stored thereon, which, when executed by the processor, cause the processor to perform operations comprising: receiving a question from an asker in which the question is associated with one or more topics; selecting 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 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 a first candidate answerer based on a ranking of the plurality of candidate answerers; sending the question to the first candidate answerer of the plurality of candidate answerers through the respective communication channel of the first candidate answerer; determining 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 a second answer to the question from the second candidate answerer; and sending the second answer to the asker and information that identifies the second answerer.
14. The system of claim 13 , where selecting the plurality of candidate answerers comprises selecting candidate answerers that communicate using a communication channel of the asker.