Even with their data policies, FB (and any ad platform) enables massive data-mining by the third parties which host the ad exchanges and destinations.

And microtargeting can be abused (or just plain used, depending on your perspective) to infer only additional data by incorporating it into the 3P analysis once users click and start loading non-FB content. Microtargeting by its nature leaks information about the target segments ...

Figuring out the data FB uses to microtarget is simply a matter of buying enough ads, or getting in the middle of enough campaign-to-user relationships (as a central 3P ad exchange or tracking service).