Starting today, the address people use to get to your timeline, and send you email on Facebook, will be the same. Previously Facebook avoided using the term "email" and instead referred to their system as "messaging". If you've "messaged" someone on Facebook, you've used their email system.
Facebook usernames make it easier for people to find you or your company by entering your username as part of the URL in their browser. Your Facebook URL and your Facebook email address are now one in the same. Kinda like Google + (if you will) Over the next few weeks, Facebook will assign yours. If you've already claimed your email address you will not be affected.
Your username will have the same privacy setting as your profile name in Search, and you can always edit your search privacy settings here. Maintain your brand by being conisitent across all social platforms. Look up at your URL, if it has a bunch of numbers, you haven't claimed your "Vanity URL" yet.
If you haven't claimed yours, visit facebook.com/username.
The Markit Team