You've already hit the nail on the head. Facebook is providing a friendly user interface to do normal people stuff. To the users who aren't technically sophist

You've already hit the nail on the head. Facebook is providing a friendly user interface to do normal people stuff. To the users who aren't technically sophisticated or haven't thought deeply about any of this, sucking up their contacts and shipping them off to wherever is just not something they would have imagined, let alone agreed to. And yet they did, at least technically, when they clicked some button saying they had read 40-pages of legalese.