I first came across LinkedIn a couple of weeks ago thanks to an invite from a colleague. It's finally a sort of social networking I can actually picture myself using. Contrary to Orkut that I registered with to see what all the noise was about, and subsequently ignored, and all the other similar sites, Linked In serves a definite purpose that people are spending a LOT of time and resources doing more or less manually in the past.
Linked In focus on adding trust to requests for contact from strangers by letting common friends mediate the contact. Just as you're likely to have introduced lots of your friends to eachother for for various reasons such as shared interests, job openings etc., Linked In lets you do the same thing while automating the logistics and providing people with powerful search functionality to allow them to see which of their friends have contacts that might be useful 2,3 or 4 steps removed.
Since all requests goes through the intermediaries, it acts as a deterrent for meaningless attempt to spam people 3,4 steps removed unless you have a genuine reason to reach them, as you're likely to get your request ignored and risk alienating contacts that may be your friends, or at least valuable contacts that you won't want to lose.
So far I've found a lot of people on Linked In that might be useful for me to know, and I intend to experiment with it for a while and see if perhaps social networking is actually on its way to growing up.