That's what you think. Even if you signed up as Joe Mc Soap who lives in 1 Main Street,Townville, all it takes is for one person to use the facebook app on their phone and now facebook knows whatever the phone knows about you, that is, your name, phone number, probably one or more email addresses and with some cross referencing, your circle of friends and your geographic location (with a high degree of certainty). The probably know your date of birth since at least one person will send you a happy birthday message or put you into a birthday reminder. They will have multiple photos of you even if you don't allow tagging because people will attempt to tag you anyway. They'll have a good idea of your socio-economic status based on that of your friends. You can probably take comfort in the fact that they would know all of this even if you didn't have a facebook account since they create shadow accounts for such identities so you probably haven't done anything to compromise your privacy, your facebook using friends have inadvertently done this for you.