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 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 janedoe@facebook.com.

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.