Facebook claims the switch was made in order to be more "consistent" across the site, but this did little to appease the users irked at being kept in the dark.
"Warnings would have been nice Facebook, don't just go and change email addresses," shared Josselyn Arundell (@josselynarundel) via Twitter.
The social media giant secretively replaced email addresses listed in user profiles with new @facebook.com addresses on Monday. It's the company's latest move to draw the ire of its users, who took to the competition in order to vent.
"More stunningly bad work from Facebook," tweeted Darren Gough (@Darren_Gough).
"Good idea to get people to use it. Poorly executed!!!" added Brent Jagodnik (@DIGITALVisnry). "Here's the fix : http://lifehacker.com/5921095/facebook-just-changed-your-email-without-permission-heres-how-to-get-it-back."
Facebook initially launched its email service in 2010, a development users seemed to forget almost as quickly as they were informed. Forbes was the first to flag the clandestine switcheroo, and tech sites quickly began posting tips on how to revert back to your original email address.
But as the BBC further explains, the product itself is not the problem.
"This is a direction Facebook needs to move in — your email is a proxy for your identity on the internet and Facebook want to usurp people's pre-existing email identities with their own to help drive up traffic to its site and lock users into its service."
"The problem is the lack of transparency — it has acted without asking for members' permission first."
In fact, users might find themselves appreciating their new email address. Emails sent to a user's @facebook.com address will "appear alongside posts sent via the network's internal message system, allowing users to pick up both types of communication from the same place," according to the BBC.
"We are providing every Facebook user with his or her own Facebook email address because we find that many users find it useful to connect with each other, but using Facebook email is completely up to you," Facebook shared in a statement.
"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."
But, alas, it's the lack of disclosure that's whipped Facebook users into a frenzy.
"It reeks of the same move Google did with its Buzz product when it automatically opted people in, and users recoiled against the action," shared Anthony Mullen, interactive marketing analyst at Forrester Research with the BBC.