This rule will not apply to people who had previously activated a Facebook.com email address.
Both customizing your profile URL and activating your Facebook.com email address are optional. However, there is concern that for users who enable both features, the change could make them vulnerable to receiving unwanted email on that Facebook.com email address.
Facebook first allowed users to create a profile URL username in mid-2009. The username became the second half of the profile's URL. If user Jane Doe selected "janedoe" as her username, for example, the URL of her Facebook profile went from including a computer-generated ID number to being simply www.facebook.com/janedoe. Now, if Jane Doe activates her Facebook email address, she will have to use firstname.lastname@example.org.
Ray Valdes, a Gartner analyst, saw the move as "another step in moving beyond the walled garden."
"The idea is that Facebook users spend more and more time within Facebook, but they're also connected to the rest of the world," Valdes said.
Facebook offers members the option of having a Facebook.com email address so that they can receive messages within Facebook that were sent from external email services, such as Gmail or Yahoo. People can also send email messages from their Facebook.com address to non-Facebook recipients.
The changes will mean that visitors to a Facebook user's profile will also know his or her Facebook email address, in those cases where both the custom URL and email handle are the same. The feature could potentially result in unwanted messages for users who have more liberal settings controlling their incoming messages.
"Whenever Facebook does anything, there's always risk of user backlash, there's always a risk of privacy concerns," said Gartner's Valdes. "Whenever there's a change there's an opportunity for user error and inadvertent disclosure of information."
But Valdes noted that Facebook "moves users to the edge of their comfort zone but usually not beyond."
The company did not immediately respond to a request for comment.
Facebook will activate the new addresses over the next few weeks, according to the announcement.
Cameron Scott covers search, web services and privacy for The IDG News Service. Follow Cameron on Twitter at CScott_IDG.