Both in job hunting and filling positions. It goes both ways. Rather than trying to capture everything social it has focused on just capturing the career network and doing it well. If I'm on LinkedIn it's for a reason: Jobs.
If I'm on Facebook it could be any number of reasons all of which probing raises privacy concerns.
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.
If I'm on Facebook it could be any number of reasons all of which probing raises privacy concerns.