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But.. they do, though. You can (and people do) make a very targeted ad, then query what users matched with it, and so on until you've sufficient data for your purposes. Plus you can use their public APIs to then match their ad data with the users public information. Facebook knows this, and does not prevent it (by hiding user identifiers for instance) because it's part of their strategy.

Ah now this is sneaky. Is this widespread? Any sources?

Here's a guy targeting his roommate[0]. Previous discussion[1].

[0] https://ghostinfluence.com/the-ultimate-retaliation-pranking...

[1] https://news.ycombinator.com/item?id=8330931

By making a Custom Audience and then selectively uploading? I think you have to agree to not do this.

The audience size tricks like uploading an audience, then adding one to it and reuploading won’t work. There’s a cardinality fudging thing.

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