If you go to Your settings, under privacy settings there are a few options, including one to control who can look you up using your phone number. If you set that to "friends" then only people who are already on your friends list can use your number to find you. There's one for looking you up by e-mail too.
It worth going though all the sections in your settings periodically to review your privacy, just in case things have changed or been added with updates.