I'm not sure when LinkedIn launched it, but it now has an Answers section. This is pretty neat. First off, they bring the user in by putting the questions that people ask on the right bar next to people's profiles. I haven't played around with it enough to see how well the questions are chosen based on the profile, but I presume that algorithm will improve over time.
What the system allows users to do is simply to ask questions and then have people answer them. People who contribute more get higher rankings on the system as experts. It looks like, for now, experts are those who are top contributors for the week. This reduces the incentive for bots to do fake contributions since they'll only succeed for a week and probably get little response anyway.
Cool tool that everybody should check out.