What is a Facebook username?

You can personalize your Facebook URL (web address) by selecting a unique username. It will appear in the location bar of your browser after "https://www.facebook.com/" when you view your profile (timeline) or Page.

An example of a username is the phrase "fbsitegovernance" as used in the web address https://www.facebook.com/fbsitegovernance.

Whats My Facebook Username

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RE:
What is a Facebook username?

if it's profile, it's your facebook profile URL name.
if it's page it's your facebook page URL name

Facebook has been introduced an user name which is the subpage of facebook.. Like this

www.facebook.com/YOURNAME

this YOURNAME is user name.

You can make a user name on your facebook account by browsing account setting tab.

The email you use to make your facebook account.

the name for your page

username is the name by which everyone will know you on facebook.

The email you use to make your facebook account.