Even if a user had set their default sharing option to "friends", the bug changed the setting for affected users to "public".
Erin Egan, Facebook's chief privacy officer, was quick to apologize for the mistake, but the question remains: can we still trust Facebook?
It is still unclear how numerous 14 million profiles that were affected may have posted content privately without realizing they were sharing publicly.
Ms Egan says the issue did not affect past posts and has apologised for the mistake.More news: Posts $400M Q1 loss, revenue totals $3.09B
A Facebook post's privacy settings, with a checkmark next to the users' chosen default setting.
The problem, which Facebook says it has fixed, is the latest privacy scandal for the world's largest social media company.
Facebook said it had reverted the audience settings to users' prior preference. The affected users will then be shown which posts were marked public during the glitch. In the process of creating this feature, Facebook said it accidentally made the suggested audience for all new posts public.
VentureBeat has reached out to Facebook for more information about who may have been affected and will update this story if we hear back.More news: Top fashion awards honor Raf Simons, Supreme
"This bug occurred as we were building a new way to share featured items on your profile, like a photo". It is believed that the bug was live for a period of 10 days or more during the month of May.
The firm added that the notification is part of Facebook's broader and recent efforts to be more transparent about its product and how its handling privacy issues.
A Facebook booth is seen at the China International Big Data Industry Expo in Guiyang, Guizhou province, China, on May 27, 2018.
That meant that if the user did not notice the setting had changed, they may have posted something publicly that was not meant to be seen by a wider group of people, for example a new mobile phone number or address. That way, users can reset a post that was inadvertently set to public back to being shared just with friends if they would like.More news: Amazon is new player in live Premier League TV rights deal