A unique numeric number is associated to your Facebook profile, group, or page that is called Facebook ID. Some people though it is the ID by which you can use as username.
Every Facebook profile or page is assigned a Facebook ID, which is used to identify you among the billions of other profiles and pages on Facebook.
Every Facebook profile, business page, or group is automatically given a Facebook ID when it is created. You can easily find your or someone else Facebook ID by using our Facebook ID finder. Depending on your privacy settings, anyone with the user ID can view your profiles and pages.