Well said Mike
Points are being missed.
Linked-In is extremely good for marketing, but it will largely depend on the audience you are targetting as it will 'tend' to be business people on the site.
You can build up an audience on linked in by first creating contacts with people who you already know and trust, ideally people who are interested in your industry or subject matter. If you can find these people, then you are off to a good start. Once you have struck up these links, you can then begin to browse their connections as some of those people may also have a similar set of interests. These are your second level connections, then work on your third level and so on and so forth.
By building up a list of connections in this way, you are close to ensuring that you will be growing a relevant audience to have discussion with, but you need to discuss things with them also.
Check out what discussions these people are in, get involved if those discussions cover your subject matter and of course, try to connect to other people within those discussions.
Do all of the above and you can create a strong audience on linked in and not just a list of recruiters