As you may know, they send all of your contacts who aren't members messages saying '<person name> invited you to join their network on (linkedin|facebook|twitter)'. Sometimes they even mail to 'remind' you again a month or two later.
I don't think users necessarily realize these messages are being sent ostensibly on their behalf. In my view these large, successful companies are growing their user bases through spam.
I know - I had family members asking me 'what this linkedin thing was'.
That's when I quit.
(I don't remember giving them my password, however the link to send someone an invite is actually a link to send EVERYONE and invite, and apparently that includes the contact list they uploaded (or from my point of view, stole))
I don't think users necessarily realize these messages are being sent ostensibly on their behalf. In my view these large, successful companies are growing their user bases through spam.