I have a tendency not to like social networking sites - sites like facebook and myspace bore me - my friends tell me they just changed a diaper or just had a cup of tea.. not exactly thrilling news.. however, LinkedIn is different.. although it still has frustrating limitations (if I were in charge of that company I would have a lot of ideas for improvement and expansion!!) it does allow you to find out useful stuff.
Although the social linking feature is useful if you need a lead into a company rather than making a cold call ("I know you - you are in my third tier once removed on LinkedIn.. can you get me the phone number of the Head of IT at your company?") the most useful piece to me is the questions and answers section..
It is a great way to get your business and technology questions answered for free, yes.. but it is also a great way to find out who really knows their stuff before deciding to use a provider. And for providers (like me) it is a great way of showing that you know your stuff by helping others in the community - and that makes them more likely to use you when they need those skills.