Social media giant Facebook is starting to notify 87 million of its users whether their personal data was harvested without their knowledge by Cambridge Analytica, the Britain-based voter profiling company U.S. President Donald Trump’s campaign hired to target likely supporters in 2016.

Facebook believes most of the affected users, more than 70 million, are in the United States, but there are also more than a million each in the Philippines, Indonesia and Britain.

The company has apologized for the security breach, with Facebook founder and CEO Mark Zuckerberg acknowledging the company made a “huge mistake” by not more closely monitoring use of the data and not taking a broad enough view of the company’s responsibilities.

Facebook allowed a British researcher to create an app on Facebook on which about 200,000 users divulged personal information that academic Alexsandr Kogan subsequently shared with Cambridge Analytica.  The number of affected Facebook users multiplied exponentially, however, because of the data collected from all the friends, relatives and acquaintances the 200,000 had online Facebook contact with.

Cambridge Analytica says it only had data for 30 million Facebook users.

Zuckerberg is meeting privately with lawmakers in Washington about the controversy and then testifying publicly Tuesday and Wednesday before two congressional committees.

Facebook is sending a notice to all of its 2.2 billion users with a link to see what apps they use and instructions on how they can, if they wish, shut off third-party access to their apps.

 
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