I should add that in old scenarios where they couldn't do behavioral patterns very well, they still had specific circumstances they could track.
For example, say a specific hard drive model has a known controller logic problem that will manifest in drive just randomly not being recognized on boot any more after a specific amount of time. Imagine something like Seagate Barracuda 7200.11 controller failure, which was well understood to happen in a specific time frame after first start of the drive due to controller logic bug.
Now imagine I'm facebook. I got a tracking cookie that enabled me to see that you did in fact buy this drive on this day. I also got a tracking cookie from a page with build guides that indicates you accessed it the next day.
That means I can push very expensive, very high value data recovery service ads to you within specific time frame when this drive is expected to fail. And you will have a moment of "holy fuck, my drive is gone, my wife's photo album, my kids, my dog, my pet fish!!! Panic! What do I do?"
Go on facebook to ask and... it's right there in your face before you even get to the relevant local group. An ad for a local data recovery service. "Oh my god, thank you facebook! You saved my marriage!!!"