Facebook offers members the option of having a Facebook.com email address so that they can 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 messagesfor users who have more liberal 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.