So, how often are you on linkedin then? every two years? Or how often do you change jobs?

Depends, like I said. I've used it both in job hunting and filling positions that are open. In the case where I was job hunting I did searches on the people who interviewed me either before (if I happened to know who I was meeting with) or after the interviews.

I even searched for people I didn't interview with, but were listed as having worked for the same company with similar job titles. It's a good way gauge the caliber of the people they've hired to work there.