[jwdunne]

This is called an "audience". You can pretty much create an audience from anything.

You can build up a whole chain of audiences. For example, if you watch a video ad, you can then be targeted. If you, say, click a link as part of that audience, you can find yourself inside an even tighter targeted audience. At that point, you're considered hot. Before you know it, you could be buying a product. At which point, you're in another audience still, tempting you to buy again!

In fact, with this type of targeting, interest and demographic based targeting doesn't even come close. You can, however, target all the people, at each step in your funnel, who "look like" people who wanna buy (have the same interests, age, gender). These get fed back into the top of your targeting funnel and so it goes.

As for the UK GE, I'm not seeing this type of targeting so I think they may be stuck in the "old" interest-based model of FB advertising.

[ThrustVectoring]

This looks like a hilarious accident of enabling scammers to target vulnerable people through the magic of gradient descent. Nobody would want to look like they're intentionally targeting multi-level marketing schemes to special education students, but if the students happen to "become part of an audience" and "people like your audience" happen to be special education students, then you get all the benefit of exploiting them with none of the terrible optics.