Your friends, colleagues, and family likely have you in their contact lists, so Facebook knows you are missing from its graph. In lieu of an active profile they

Your friends, colleagues, and family likely have you in their contact lists, so Facebook knows you are missing from its graph. In lieu of an active profile they maintain a shadow profile for (at least) your phone number, and whatever additional information people who know you have unwittingly provided to Facebook and its daughters (e.g., WhatsApp). Facebook knows us by our social connections. We don't have to actually use it for that. As long as your browser's privacy add-ons disable Facebook's widgets (the like-button) it probably can't track your browsing habits though.