Now, let's say I own an apartment block and want to advertise for tenants, but I don't want any of them dirty muslims in my building. I can target my advertising to exclude people whose interests include Halal food.

Or, let's say I'm the propaganda department of an adversarial nation state, and I want to stir up communal tension. All I have to do is find people with an interest in Halal food, and target them with ads exhorting them to join other Christians in the next Crusade against the Moors. Or whatever. This was actually a common technique in the 2016 election: identify voters who, via their social connections and profile interests, are likely on the fence about Clinton vs. Trump, and also are a bit homophobic -- then target them with "Queers for Hillary" advertisments.

It's shocking how easy it is to weaponize this stuff.