The app vacuumed up not just the data of the people who took it,but also - thanks to Facebook's loose restrictions - data from their friends,too,including details that they hadn't intended to share publicly.
Facebook later limited the data apps can access,but it was too late in this case.
Zuckerberg said Facebook came up with the 87 million figure by calculating the maximum number of friends that users could have had while Kogan's app was collecting data. The company doesn't have logs going back that far,he said,so it can't know exactly how many people may have been affected.
Cambridge Analytica said in a statement on Wednesday that it had data for only 30 million people.
AP