The company claimed that it has acted to make communications more "consistent" across the social network.
But many observers feel that this is part of plans to draw people into using the Facebook email system rather than the services of other providers.
Newly-listed Facebook would benefit from this by driving more traffic to its page and therefore boosting advertising revenue.
The social network announced plans to move to its own email system in April.
It said in a statement at the time: "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."
All Facebook users have an @facebook.com email account created for them, whether they use it or not.
Facebook is now automatically posting these addresses to users' profiles and also displaying them as their default email address.
The move actually started rolling out a few weeks ago, but has attracted a torrent of criticism this week after it was reported on technology sites and blogs.
Blogger Gervase Markham, among the first to highlight the change, accused Facebook of having "silently inserted" itself into "the path of formerly-direct unencrypted" email communications of its users.
"In other contexts, this is known as a Man In The Middle (MITM) attack," he wrote, referring to a popular tactic used by hackers to intercept electronic communications.
"What on earth do they think they are playing at?"
Posting on Twitter, freelance tech journalist Ron Miller tweeted: "Speaking of hating your users. Facebook forces everyone onto its email system. Really, Facebook?! Really?!"
Very few users have supported the move, predominantly because it was done without full consultation with the site's 900m members.
Josselyn Arundell, from Manchester, posted on Twitter: "Warnings would have been nice Facebook, don't just go and change email addresses."
California-based Brent Jagodnik added: "Good idea to get people to use it. Poorly executed!!!"
Analysts have also warned that the move could backfire for Facebook in that it risks alienating people from its own email system.
Anthony Mullen, interactive marketing analyst at Forrester Research, told the BBC: "It reeks of the same move Google did with its Buzz product when it automatically opted people in, and users recoiled against the action.
"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."
Anyone wishing to undo the change can access the About section on their profile page and then click Edit next to the contact information.
This then opens up an option to 'hide' the Facebook email address from your Timeline, and replace it with another email address if you so wish.