Facebook is notifying people who haven't yet picked a username that they had better do so, because Facebook is switching all timeline addresses over to a Facebook.com/username structure for consistency. That username is also good for receiving Facebook messages via email.
For those who have already chosen a unique username, you are good: That is now your timeline address and your Facebook email address.
For instance, referring to the Jane Smith example Facebook provides: The user can direct friends to her timeline (assuming she has granted access to them) using facebook.com/jane.smith.19, and she can receive notes from friends who address their emails to firstname.lastname@example.org.
If you haven't chosen a username, now is your chance to do so, rather than getting stuck with whatever Facebook assigns you. The link at the bottom of the April 12 announcement will take you there, or you can just wait until a note pops up on your timeline.
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