Facebook has confirmed it believes up to 87 million people had their data harvested by Cambridge Analytica through a personality quiz app called 'This Is Your Digital Life'.
The company announced it will start to notify users with a detailed message appearing on the news feed of those who might have had their data shared.
Facebook says more than 70 million of the affected users are in the US, though there are over a million each in the Philippines, Indonesia and the UK.
But, users do not have to wait to be notified that their data was shared with Cambridge Analytica - a tool shows users to quickly find out for themselves.
Here's how to find out:
Log in to Facebook and go to facebook.com/help.
In the search bar - enter the word 'cambridge'.
Select the top search result: 'How can I tell if my information was shared with Cambridge Analytica?'
On the page that loads - a box saying: 'Was my information shared?' will appear.
It will confirm whether or not your data was accessed through the personality app on yours or a friend's account.
There is a link at the bottom where users can review and update the information they share with apps and websites.
Facebook CEO, Mark Zuckerberg is set to testify before a joint session of two Senate committees on Tuesday and Wednesday.