If you are an administrator of a Facebook Page, you will be able to choose your username at [facebook.com
...] just like a user selecting a username for their profile (timeline).
I have seen that even short URL (unique) can not be added.
Error message is the following:
greatweb is not eligible for a username at this time. In the future, Greatweb will be able to set a username.
Is this true and not working anymore? I thought this will be temporarily issue at Facebook but this is all the time.