Perfect example is Facebook. You don't have to be on it for them to know your phone number. If 2 of your friends have the "Share your Contacts with Facebook" op

Perfect example is Facebook. You don't have to be on it for them to know your phone number. If 2 of your friends have the "Share your Contacts with Facebook" option turned on, chances are Facebook has your phone number/whatever else your friends store on their friend about you.