What are usernames?
Usernames and User IDs are part of your Public profile. We use this information to help people find you and to organize your information internally.
We use your username to create a custom link (a Facebook URL, such as www.facebook.com/username) to your profile that you can give out to people or post on external websites. Though Usernames are not the same as your name or Page name, they are typically variations on your name. For example, someone named Jane Doe might choose "jane.doe" for her username so her friends can visit her profile by going to www.facebook.com/jane.doe. Learn how to customize your profile username.
A User ID is a string of numbers that does not personally identify you, while a username generally is some variation of your name. Your User ID helps applications personalize your experience by connecting your account on that application with your Facebook account. It can also personalize your experience by accessing your basic info, which includes your public profile and friend list.
Your Username or User ID can be used to access information about you through Facebook. For example, if someone has your username, they can type “facebook.com/” plus your username into their browser and see your public information, as well as anything else they're in the audience for. Similarly, someone with your Username or User ID can access information about you through our APIs, such as our Graph API. Specifically, they can access your public information, along with your age range, language and country.