I think this is mostly not true. The missing ads here are high-end (as advertising goes) brand ads. This isn't the blender you just looked at on Amazon following you around or weird tricks to remove belly fat. These brand advertisers are not just looking for lots of people to see their ads, there are lots of places they could buy that more cheaply, they are looking for prestige, trust, respectability, etc. This is why you see these ads on the NYT but not on gun forums. Ad space on Twitter is worth far more if the advertisers have guarantees about what their ads might be shown next to (and if Twitter has a good reputation).