There seem to be three major populations on LinkedIn:
1. High-powered executives
2. HR people, recruiters, and others in the employment industry
3. IT people and related technologists

For the past week or so I've been fleshing out my presence there, and I'm a little surprised at the big holes in the membership:
1. Scientists. I used to work at Argonne National Lab, which employs about 900 Ph.D.'s. And yet the majority of users who turn up under the search term "argonne" are in the IT department. Of the dozens of scientists whose email addresses I still have, only two were on LinkedIn. (And they haven't answered my requests!)
2. Sales. These people are consummate networkers ... why aren't they here?
3. Almost everybody else. Technical professionals, managers, the lot.

It's an odd sort of trimodal demographic.