I joined Linked in a couple of weeks ago as a student invited me. I had always intended to join as I perceive it to be a professional social network rather than just a social network. It has the sense of being a little more selective than facebook - but that's probably because it doesn't do very much apart from connect people ("professionals") and show their twitter feeds.
What was interesting for me was that most of my Music Business lecturers and media lecturers are on there already - I still get a bit of a buzz when I'm invited to connect to them though.
So I'm already automatically separating Facebook as friends and Linked in as professional....
I've even added a personal photo to my profile (very unusual for me)