If you're trying to encourage knowledge sharing within organizations, two strategies seem to work well.
The first is a competition strategy. Q&A-orientated sites do well here. Members are effectively in competition to generate the best idea. This works well in competitive, individualistic, cultures with a low sense of community and porous borders between social groups (i.e. a newcomer could become popular quite quickly).
The key to this is ensuring that the borders between social group are porous. It should be clear that you can rise up relatively quickly by sharing your best information and impressing other people.
The second is a collaboration strategy. In organizations with a strong sense of community, mutual agreement on goals, or with low levels of mobility between layers, a collaboration strategy works best.
The key here is finding people with the time to submit their responses. They want to help, but they're very busy. You have to involve people at a deeper level in the goals of the community and the problems it wants to solve. You have to create a very strong desire to participate to help the organization. That's not easy.
The challenges of both strategies
The problem with the former is generating the questions to be answered. People don't submit questions because it undermines their being the sole expert on the topic. This means you need to set topics or an agenda that stimulates activity which can be answered.
The problem with the latter is getting responses to questions and soliciting visits in the first place. This means having clear goals for the community, documenting the group history, and more traditional team-building activities so people make the time to help.
Neither strategy is superior, but you probably need to pick one.