Facebook users were venting anger over a move changing users' default email address to the one operated by the leading social network.
"You can thank Facebook for making that change without telling you," said security consultant Graham Cluley of Sophos.
Facebook indicated in April it would be "updating addresses on Facebook to make them consistent across our site."
Bloggers began noticing over the weekend that their contact information had been changed in what some called a "Gmail Killer."
"Facebook silently inserted themselves into the path of formerly direct unencrypted communications from people who want to email me. In other contexts, this is known as a Man In The Middle (MITM) attack. What on earth do they think they are playing at?," said Gervase Markham on his blog, "Hacking for Christ."
Cluley said on his blog, "The social network didn't make clear that it would also be making the @facebook.com email addresses the default address displayed to your online friends.
"Clearly this all part of the site's plan to get more people using the @facebook.com email addresses, thus making the social network even harder to extricate yourself from."
The moved coincided with a decision by Facebook to postpone a feature called Find Friends Nearby, a location service that identifies other Facebook users in the vicinity.
Facebook, which has more than 900 million users worldwide, has been criticized in the past for changes, especially on privacy settings.
The email shift appeared aimed at getting users to remain within the Facebook ecosystem, where they can be hit with targeted advertising, instead of moving to rival services from Google, Yahoo! and others.
Facebook users can restore their original e-mail addresses by editing their profiles.