Guidelines for creating a custom username or screen name on Facebook
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When you create a custom username for your Page or profile, keep in mind:
You can only have one username for your Page or profile, and you can't have a username that is already being used.
Usernames can only contain alphanumeric characters (A–Z, 0–9) and periods ("."). They can't contain generic terms or domain extensions (e.g., .com, .net), including country extensions (e.g., .ie, .de, .ca) .
Usernames must be at least 5 characters long.
Periods (".") and capitalization can't be used to differentiate usernames. For example, johnsmith55, John.Smith55 and john.smith.55 are all considered the same username.
Usernames shouldn't impersonate someone else.
Your username must adhere to the Facebook Terms
.
Usernames for Pages
Keep in mind that you must be an admin
to create or change the username for a Page.
You may not be able to immediately create a username for a new Page if you’ve recently created several new Pages. A Page’s username can also be removed because of inactivity.
