I have been on linkedIn for over a year now. I was invited on and thought I would give it a go. I don't use it alot but do follow some discussions via the CILIP group. My page:
Obviously compared to facebook there are less people on it but it is more business/work orientated. I probably ought to use it more but do find the connections part annoying - it doesn't seem to allow for the fact that you might have a colleague not directly in your workplace and not just working on a project. It asks you where you worked together and if it is not your current place the dates you worked together. If you have nothing to put there you have to drop down to "friend" and then you are entering facebook territory.
I find myself a bit ambivalent about it to be honest. I get email updates on discussions from the CILIP group and sometimes I connect to follow it further but not hugely engaged.