There are many guides to make sure your Facebook privacy settings are what you want them to be. This privacy concern is actually about a piece of information that can be found via your Facebook timeline URL and used to discover your presence elsewhere online. The information is the user name in your timeline’s URL.
But, not everyone has this. Mine, as you can see above, is simply my name because that’s what I used when I signed up for Facebook. If you used a username to sign up for Facebook (waaay back when that was common practice), then you should be aware that it can be used to search for other accounts you have used the same username for. Maybe you don’t have anything you want to keep private, perhaps just an old SoundCloud account. But if you are interested in monitoring your online presence closely, now you know this trick can be used.