Do professional social networks work ?

I have used the social networks since 2004 (or so) when I received a first invite to join Plaxo, they were claiming to avoid keeping track of your contacts' moves among companies, email and phone number changes and so on.
Later, around 2005, I signed up in Linkedin where I am quite active with more 500 contacts after five years as a user and every now and then, somebody asks me "what is the point of these professional social networks? Do they work?" Well, here is my answer.
To me, business is most of it, about contacts, a good product is only the first step, then you need to build a trusting relationship between the seller and the buyer.
Same applies to professional opportunities, you count first on the people that have worked with you or near you or from whom you have references through a common friend or (ex) colleague.
Professional social networks widen the connection line of sight, there you have real contacts, people you really have met face to face at some point as well as online contacts from whom you know their activity, their experience, skills, contacts, travelling activity and some others.
The further a reference is the less trust it creates but all connections are useful, at least to keep track of where they are working...