What is a Facebook unique ID
Facebook unique identification numbers or Your User ID is a string of numbers that doesn’t personally identify you but does connect to your Facebook profile. You have a User ID automatically, whether or not you choose to create a username. Anyone with the user ID can see your profile, including any public information.
It helps other applications personalize your experience by connecting with your Facebook account. When you allow apps to connect with your Facebook account, they can use your user ID to see public information, like your public profile and your friend list. When you run into issues with an app or game, your User ID can help the developer better investigate the problem to understand and address your specific concerns.
Facebook ID, is the specific number that was assigned to you when you first joined the site. Facebook assigns each new user a unique identification number. You use this identification number in the Facebook URL to find a user. The user’s profile displays and you can submit a Facebook friend request or read the user’s Wall posts using the identification number in the URL’s “ID” query string.