How many invites to refer one member of your network to another have you received? Isn't that the real measure of the value of a social network site, e.g. whether it enables connections to be made that would otherwise be impossible?
My network reflects the relationships I already have. If it doesn't enable something more, it's just a glorified contact book (admittedly one in which I get an interesting glimpse of my friends/colleagues job aspirations trough their assertions as to who they are interested in hearing from :-)).
I like LinkedIn and I enjoy the feeling of nurturing my set of connections through invitations to join, but I can't say I've benefited from the supposed 'network effect'.