Harmless? Every page which has anything Facebook-related on it will be fed your Facebook-related cookies on every visit. Any Facebook-related code on such pages will have access to anything Facebook-related stored in your profile (cookies, local data, etc). Since everyone and his dog felt the need to add 'social' buttons to their sites this includes a sizeable portion of the 'net. You might not use Facebook much but rest assured that Facebook uses you. You are part of their product after all...