The world’s largest social network has long provided members with an email address, but until now did not force them to display it in preference to a rival service, such as Gmail.
Facebook said the change to email addresses, which was made over the weekend without notice, was designed to “make them consistent across our site."
"In addition to everyone receiving an address, we're also rolling out a new setting that gives people the choice to decide which addresses they want to show on their timelines," it said.
According to observers, the move is also meant to encourage members to make Facebook the centre of their online communications and spend more time on it. The firm introduced its free email service in 2010 to compete with Gmail but it has so far failed to make much impact.
Members are able to manually restore a non-Facebook email address to their contact details, via the “About” section of their profile settings. Many were, however, annoyed by the firm’s unilateral action, which was spotted by blogger Gervase Markham.
"I happened to visit my Facebook profile, and noticed that they had changed the displayed email address to gerv.markham[at]facebook.com!," he wrote.
"The old one was still in the database, but it had been hidden. Email to the Facebook address is forwarded by Facebook to the other one, so it ends up in the same place.
"What on earth do they think they are playing at?"
Critics said it echoed the way Facebook has previously introduced controversial changes, such as when it turned on the “Timeline” feature by default last year, displaying much more historical data on profiles.
