I used to think of LinkedIn as a glorified Rolodex. Although it was useful to maintain contacts with business associates, there was little value that a participant derived from this social network. Although it was nice to see how many contacts you had, other than consultants who were trying to sell themselves by answering questions, the value to the discussions was almost nill. Thus,the question ... what is changing my mind? The answer is their groups (and groups search / updates). My contacts, although great people, were generally folks whose work and "personal stories / research" I already knew. LinkedIn was not helping advance my knowledge.
While this observation almost sounds trite, and too obvious. If you are involved in benchmarking and researching new ideas, the groups search capability now allows one to explore domains and people outside of your own contacts. It is exactly that need which one needs filled ... to get outside of what and whom you know ... to move forward.
At present I have about 250 contacts. By using the new weekly update email, I find it valuable to see what groups my network of contacts is linking to. As an example, I recently found the Web 2.0 and Mapping Social Network group. Some research revealed that this group (many similar titled groups exist) actually has an active membership that wants to exchange knowledge. Although I don't work in Silicon Valley, via the groups web site I am able to participate in their meet-ups and watch some of the quick demos of new ideas.
As always, you get what you put into any organization, but I am now able to find new groups with which to exchange ideas. I always scan the membership up front, checking the kinds of companies and job titles from which the membership is taken.