Starting Monday, Facebook will contact the 87 million users who might have had their data shared with Cambridge Analytica.
Those affected will get a detailed message on their news feeds.
In addition, all 2.2 billion Facebook users will receive a notice titled "Protecting Your Information" with a link to see which apps they use and what information they have shared with those apps.
If they wish, they will then be able to shut off apps individually or turn off third-party access to their apps completely.
The company say 70 million of those affected are in the US, though there are also over a million each in the Philippines, Indonesia and the UK.
Reeling from its worst privacy crisis in history...