Today (April 9), Facebook will reach out to 87 million users of the social network that had their data harvested by Cambridge Analytica, one of its biggest data breaches ever.

Most of these users are based in the U.S (approximately 70 million), with over one million in the UK, Indonesia, and the Philippines, as well as 310,000 Australians also thought to be affected. These profiles will received a notification and message explaining the issue on their news feed.

In a report by the Observer, it was first revealed that data company Cambridge Analytica had scraped data from millions of Facebook profiles to create an algorithm that would predict how they vote and influence them with targeted ads. This firm worked with Donald Trump’s presidential campaign team, and was linked to the Brexit campaign. Facebook was first informed of this in 2015, but kept quiet.

Cambridge Analytica, according to the Guardian, claimed that the number of users affected would be closer to 30 million, but Facebook founder Mark Zuckerberg says they calculated the approximate number according to the number of friends profiles would have had during the app’s run.

Facebook will also send a ‘Protecting Your Information’ note to all of its 2.2 billion users – it will include a link to see what apps they’ve authorized and the information the apps have access to. From there, they can opt out of third party access.

Speaking with reporters from the New York Times, the FB CEO agreed that the social network could have done more to oversee third party apps, and he accepted blame for the leak. Facebook as a company came out to support proposed legislation in the U.S that would make social media networks disclose identities of anyone buying political advertisement space.

Zuckerberg is expected to meet with lawmakers in the U.S today, and will also attend and testify at Congressional hearings about the data harvesting.

Read back on our interview with Christopher Wylie, the Cambridge Analytica whistleblower, here.