I can only really speak to Amazon and Facebook (via the web) because my uBlock configuration nips almost everything else, completely[0], and most advertising from those sites, as well.
But when an advertisement does slip through, it is frequently for a service I already subscribe to or a product I already purchased. In the latter case, I've had times where I've purchased something on Amazon and had the same product show up within a few minutes of the purchase as an advertisement on Facebook. So ... that was wasted marketing spend.
On the flip side of that, I'm certain Amazon knows who my girlfriend is and correlates our searches. There have been a handful of times when I was discussing gift ideas and she'll pull up a product and hand me her phone[1]. Sure, they pick it up when she sends me a link from the app or does some other action that can be correlated (i.e. we're both on the same WiFi, our phones are both reporting the same location[2]) but it's happened enough that I've started paying attention. On one occasion, she handed me her phone, we had no other digital back/forth and my phone was charging in the car (off, because I let the battery die). Later that evening, I'm on my laptop and that product was on the home page. There are other explanations, sure, but it'd be pretty easy to see that we frequently share one of two IP addresses when visiting their sites and start tracking those accounts as affiliated in some way for gift-giving purposes.
[0] I see product recommendations on Amazon and for some reason more than a few sponsored posts and the like on Facebook -- I don't use their mobile apps.
[1] I really, really hate using mobile apps.
[2] I don't actually know if the Amazon app tracks location ... I don't have it.
// FWIW: I really did laugh out loud at "Statistical Astrology" and I'm positive I'll be using that statement in the future.