Tony Karrer has a post about Discussion Creation Among Bloggers - LinkedIn, Blogging and Discussion Groups where he writes about he interaction of tools involved - Social Networking, Blogging and a Discussion Group. Tony asks three questions:

• How do you use LinkedIn?
• How do you use Discussion Groups?
• What’s the interaction between them?

I use LinkedIn as a networking tool, to build and maintain a network of contacts. My definition of contact, if I do not already know them, is where both of us can benefit from connecting. LinkedIn helps me keep track of people regardless of where they work. LinkedIn also makes me visible, my profile is searchable through search engines like Google and people can find me inside LinkedIn using their network.

Since LinkedIn does not have the functionality of discussion groups I have joined groups at Yahoo Groups like LinkedinBloggers, LinkedInnovators, LinkedinEurope, LinkedinIT and MyLinkedinPowerForum. The reasons are to get into discussions (to share and gain knowledge), to be seen and to get more contacts.

I am also a member of XING. Since XING contains it own discussion groups it is easier to see the connection between the networking and the discussions. Within Xing are both my contacts and my discussion groups, it is much easier to keep track of intersections - finding people who share my interests.

The third question is about the interaction between networking and discussion groups. I would say that on LinkedIn I can see someones profile but they are often rather condensed and kind of a snapshot. In a discussion group I get a chance to interact with people and see them interact with others. It gives me more information about a person, about his/her knowledge and willingness to share and learn.

Update.
LinkedIn Anwers is mentioned in comments. I do not see that as a discussion group since you can ask a question, then any member can answer once. The only way to continue from there is by mail and one on one.