Someone can use your username or user ID to access into your Facebook account. But unlike usernames, users ids are hard to remember and not human friendly. Normally your Facebook account URL will be look like this:

http://www.facebook.com/profile.php?id=1233731615

As you can see, above URL is not user friendly. So you can use a short and simple URL for your Facebook account creating an username. Generally, a short URL is easy to remember and very convenient to market your FB profile. So shorter Id holders often get more likes, comments, and reviews than longer ones.

For example this is my Facebook account URL:

https://www.facebook.com/lasanthabk

To add your own Facebook username follow the easy steps given below.

1.Login to your facebook account.

2.Go to https://www.facebook.com/username/

3.You can see a screen like below. Click on "More".

4.Now enter your desired username as the example below and click on "Check availability".

If your username available you can get it clicking on "Confirm".

You are done. Now if someone has your username, they can type "facebook.com/yourusername" into their browser and see your public information, as well as anything else they're in the audience for.