How can I add a username in logging-in on facebook instead of email address?

[Justin]

Well at first, me as a normal person, always uses facebook like everyday. And we log in using our email address and entering our password. But when I see my cousin logging in on facebook, instead of typing his email address, he types like a username or phone name or something. He types 'calizoo' instead of typing his email. And still he can log in. I tried asking him but he won't tell it to me. Of course, I want to know how that happens because, it is kinda "time-saving" and less effort instead of typing your whole email all over again everyday. I tried searching it on google but I can't actually find the right answer. So if any of you professional out there. I REALLY NEED YOUR HELP.

[Goos09]

Here is a link from facebook itselves.

To be honest i googled for 2 seconds and found this already, so I guess you didn't google sufficient enough.

Anyways, here ya go