I see these waves of ads for particular categories often on Facebook. Low-alcohol or non-alcoholic cocktails are still going strong, though they've been eclipsed by ads for a very particular kind of faux-athletic shoe (pretty sure they're exactly the same shoe even though it's a dozen companies). I have to admit, after a while I sometimes do get a bit curious. The frequency itself makes some part of my brain think this must be A Thing, and that I must pursue The Thing, even though my rational brain knows it's entirely otherwise. It's a powerful effect. Whether intentionally or not, I think Facebook's ad-selection algorithms create these "category waves" even across multiple products and vendors because they do work. Therefore it's not surprising that such a wave related to the most controllable of minds is the biggest of all.