If a user has activated Login Approvals, attempts to access the account from an unrecognized device and doesn't have the authorized cell phone at hand, he would remain locked out until he logs in from an approved device.

Facebook may consider incorporating other technology to Login Approvals beyond SMS, but it is holding off on it until those other forms of two-factor authentication become simpler.

"One challenge in building login approvals was balancing security and usability. Similar features on other websites require you to download authentication apps or purchase physical tokens to act as your second factor. These are good approaches, and we're considering incorporating them in the future, but they require a lot from the user before being able to turn on the feature," wrote Andrew Song, a Facebook engineering intern, in the blog post.

People's user names and passwords are stolen in a variety of ways, including through phishing scams, by intercepting unsecured Wi-Fi communications and with key-logging malware.

The Login Approvals feature can be turned on by going to the Account Security section of the Facebook account settings page, the company said.