Facebook said there is no evidence its employees abused access to this data. The company said the passwords were stored on internal company servers, where no outsiders could access them. However, privacy experts suggested that users change their passwords.

The security slip left the passwords readable by the social networking giant's employees.

The issue was first reported by security researcher Brian Krebs, who published a blog post-Thursday detailing that Facebook employees built applications that captured the passwords of users and stored them as plain text, meaning a password would be readable just the same as it is entered to log in.

The blunder was uncovered during a routine security review early this year, according to Canahuati.

"To be clear, these passwords were never visible to anyone outside of Facebook and we have found no evidence to date that anyone internally abused or improperly accessed them," vice president of engineering, security, and privacy Pedro Canahuati said.

"As part of a routine security review in January, we found that some user passwords were being stored in a readable format within our internal data storage systems," Pedro Canahuati, vice president of engineering for security and privacy at Facebook, wrote in a blog post. "This caught our attention because our login systems are designed to mask passwords using techniques that make them unreadable."

Most companies encrypt passwords to prevent them from being stolen in the event of a data breach or used for nefarious purposes by company employees.

The incident reveals yet another huge and basic oversight at a company that insists it is a responsible guardian for the personal data of its 2.3 billion users worldwide.

By storing passwords in readable plain text, Facebook violated fundamental computer-security practices. Those call for organizations and websites to save passwords in a scrambled form that makes it almost impossible to recover the original text. The blunder was uncovered during a routine security review early this year, according to Canahuati.