Facebook replaced the email address users chose when they signed up with a facebook.com address.
The Facebook email accounts allow users to communicate with outside email addresses via Facebook.
The changes were first pointed out by bloggers over the weekend, leading to complaints from users.
An email message to Facebook representatives seeking an explanation was not immediately returned.
The company said in a statement in April that it was "updating addresses on Facebook to make them consistent across our site."
Users are free to restore their former email addresses.