Comment by JumpCrisscross

"But 'Download Your Data' hardly tells you everything Facebook knows about you. Among the information not included:

- information Facebook collects about your browsing history;

- information Facebook collects about the apps you visit and your activity within those apps;

- the advertisers who uploaded your contact information to Facebook more than two months earlier;

- ads that you interacted with more than two months prior.

Download Your Data is particularly spotty when it comes to the information Facebook taps to display ads."

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TL; DR When you get too close to their business model, Facebook clams up.