Hans,
I am not sure what your question exactly is and I do not want to seem like I am trying to promote Linkedin so I will try to stay on the facts.
I used Linkedin as a way to reconnect with past collegues which I lost track (either because they changed jobs or I did etc) and create a kind of address book which always stays current (as people tend to update their Linkedin account everytime they change jobs or e-mail address -you can only see e-mail address for people that you are connected-).
For other people which I might not know but I share the same interests I either join a group (i.e. proteomics, mass spectrometry, Biotech & Pharma Professionals Network) or in the case of Chromatography and Systems Biology there was no group available so I created them. In this way you are able to see who is out there, their interest etc.
For those looking for a job or like to keep their options open, Linkedin has a lot of recruiters as well, but you can specify in your interests if you do not want to be contacted for potential jobs...
In the case of the Linkedin Chromatography Forum group, it has some 150 people joined.