Most of your friends, family, coworkers, etc. have your e-mail or your phone number (or both). Some (most) of them use facebook, and a significative portion of them share those information with facebook, so even if you never subscribe to the service, they know a few things about you (your name, phone number, e-mail address and some of your acquaintances), and they know they don't know anything else about you (which is some sort of information too).