As many as 14 million Facebook users had their posts shared those other than who they intended.
Facebook claims that glitch for 10 days last month switched privacy settings to "public" for millions even if they had their content set to "friends only".
"We have fixed this issue and starting today we are letting everyone affected know and asking them to review any posts they made during that time," Erin Egan, Facebook's chief privacy officer, said in an emailed statement.
The disclosure is just another round of shady dealings from Facebook, still facing backlash that political consultancy Cambridge Analytica gained information on 87 million users without their consent.
Facebook is also under fire for its dealings with Chinese companies.