Yup. And that very reason is why many people don't have social networking accounts. You can control what you share, but you can't control what your friends share.
How not having a social networking account helps with that? Not only you still can't control what your friends (that is, the real-life ones) share, now you're the last to know if they share something about you.
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.
