Networking sites as knowledge tool

Like so many others, I also have an account on linkedin but so far was wondering if its really worth spending time on. After all, adding countless people into your network is not going to do a great deal of good to you personally or professionally unless a networking site enables and even encourages frequent interaction with each other. e.g. I have few contacts in my list to whom I have not interacted ever since I added them into my network.

However, recently I have started using questions and answer sections a lot. For a professional site like Linkedin, I think this is the biggest USP. Have a look on the range of questions asked and answers given by people coming from different industry, culture, geographies. This is pure practical knowledge available at will to EACH member.
e.g. being someone from outsourcing industry, I had a look on the questions asked for this industry and I am amazed by not only the questions, which of course prompt you to think differently and widen your horizon, but also by the answers from a variety of people. And as you start participating in discussions, you will invariably have more people in your network and I think that's real power of networking sites like linkedin - not merely to add people to your network, but to encourage them interact on common issues.
So for me, the power of linkedin is not only networking possibilities, but its use as a knowledge tool or even a innovation tool due to shear range of thought process you get access to through it.