Last post I was talking about how I don't use LinkedIn to ask questions of my network and then went on to list some things I don't ask. Harry raised some other questions in the comments, and then I went away and thought about it.
I think there are different needs and different networks for different questions.
Harry's questions, and most of the questions on the AAAS board are of the "how do I break in to area X from area y" where Y is frequently post-doc/grad school and X is "other". The answers I have mostly been looking for are "how do I get ahead in this industry". I think those require very different networks and answers.
The "how do I break in" really sort of implies that you aren't already in the field and you need to build a new network. My questions are all of the type of within field.
In my own career, I had built a decent network of PI's etc... to get a post doc There were informal offers out there. I didn't take it further because I knew I didn't want to do that. SO - I flailed about and eventually got lucky at my first company.
The part where I flailed is the part where boards and blogs are helpful. That is the part where you see the questions on boards.
Once you have taken that first step, then you start to build your network and I think then the questions move toward more as I am describing (personal network, not a "public" one).
That implies that I think the people asking the public questions of "I am a grad student...how do I get a post doc" didn't do a very good job of building a network when they were a grad student!
Both have value when used at the right time.