Facebook is automatically posting those @facebook.com addresses to users' profiles and displaying them as the default e-mail address, CNN reported Monday.
Facebook has been rolling out the change for "a few weeks," the social networking site told CNN.
However, many users said they weren't aware of it until blog posts and news articles began to appear Monday.
One of the first bloggers to draw attention to the change was Gervase Markham, who expressed outrage at the site's action.
"Facebook silently inserted themselves into the path of formerly direct unencrypted communications from people who want to e-mail me. In other contexts, this is known as a 'man in the middle' attack," he wrote, referring to a tactic hackers use to intercept electronic messages. "What on earth do they think they are playing at?"
Facebook for its part seemed surprised at the negative reaction to its decision.
"We basically defaulted to show your Facebook address as we rolled this out, just to keep it consistent for everyone," Meredith Chin, Facebook's manager of product communications, told CNN.
While users can change the setting to display whatever e-mail address they like, by default the visible address will be an @facebook.com listing.