The most prominent example of this in the real world was the (or similar tools like "Marauders Map" in the past). These tools rely on a discrepancy between what users think is private and what is actually public.
If you're looking for alternative tools to analyze your Facebook friends' connections, there are several options available, including: facebook friend mapper chrome extension
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L.O.C : A toolbox for bulk-changing post privacy or scanning interactions. Risks and Warnings The most prominent example of this in the
Facebook eventually patched this vulnerability by changing how mutual friend data is exposed to third-party scripts, rendering the extension obsolete. Security and Privacy Risks of Using Such Extensions there are several options available