The news marked the latest in a series of privacy embarrassments for the world's biggest social network, which has faced a firestorm over the hijacking of personal data on tens of millions of users and more recently for disclosures on data-sharing deals with smartphone makers.
Erin Egan, Facebook's chief privacy officer, said in a statement that the company recently "found a bug that automatically suggested posting publicly when some people were creating their Facebook posts."
Facebook said this affected users posting between May 18 and May 27 as it was implementing a new way to share some items such as photos.
That left the default or suggested method of sharing as public instead of only for specific users or friends.
Facebook said it corrected the problem on May 22 but was unable to change all the posts, so is now notifying affected users.
"Starting today we are letting everyone affected know and asking them to review any posts they made during that time," Egan said.
"To be clear, this bug did not impact anything people had posted before - and they could still choose their audience just as they always have. We'd like to apologize for this mistake."
Facebook confirmed earlier this week that China-based Huawei - which has been banned by the US military and is a lightning rod for cyberespionage concerns - was among device makers authorized to see user data in agreements that had been in place for years.
Facebook has claimed the agreements with some 60 device makers dating from a decade ago were designed to help the social media giant get more services into the mobile ecosystem.
Nonetheless, lawmakers expressed outrage that Chinese firms were given access to user data at a time when officials were trying to block their access to the US market over national security concerns.
The revelations come weeks after chief executive Mark Zuckerberg was grilled in Congress about the hijacking of personal data on some 87 million Facebook users by Cambridge Analytica, a consultancy working on Donald Trump's 2016 presidential campaign.
