What that means is that the email address you had listed as your main contact address (such as a Hotmail, Gmail or Sympatico address) is now hidden to friends who visit your profile page.
In its place, users have been assigned an @Facebook.com address that routes email directly to Facebook Messages.
The change prevents users from using that Hotmail or Gmail address to contact other users outside of Facebook — helping to keep Facebook's captive audience from drifting off the site.
Changed last weekend
The email change was first spotted by bloggers on the weekend. After media outlets caught on, Facebook users began to complain in droves, with many likening the change to Facebook's recent move to steer users to its Timeline feature — an alternate way of displaying postings that irritates some people.
For those people who want their regular (and now hidden) email address displayed on their profiles, rather than their new @Facebook.com address, the switch back is easy.
To go back to your original address, click on the "about" section of your profile. Once there, look for "contact info" and click on the edit icon in the right hand corner. There, you can change who can see your email address and choose to highlight which email address they can see.
Facebook didn't say why it made the email switch, but said in April it was "updating addresses on Facebook to make them more consistent across our site."
"Ever since the launch of timeline, people have had the ability to control what posts they want to show or hide on their own timelines, and today we're extending that to other information they post, starting with the Facebook address," Facebook spokeswoman Jillian Stefanki said in an email late Monday.