I have never joined facebook, and am thankful for this. But I was suckered into joining linkedin, and when I saw they had uploaded my contacts list from my gmail account, I had to leave. Still not quite sure how they did that.
As you may know, they send all of your contacts who aren't members messages saying ' 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.
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))