Realistically, in a simple statistical way, plain "coincidences" have a significant "expected value". I.e., if you simply take the billions of people across the planet, and look even across a single day, lots of coincidences occur.
Now, add in psychological effects - "synchronicity", "frequency illusion" ("Baader-Meinhof"), "recency illusion", confirmation bias, etc... I'd expect a fair bit of compounding*.
Then, add in simple use of statistics, statistical inference, etc. and basic tracking of user navigation around the web, on a given website, etc.
I've had these experiences, perhaps one or two times a year, on average. Experiences where I was VERY surprised by ads presented. Experiences that would easily suggest a microphone must have been on when it shouldn't have been. Sometimes, I realized I'd used someone else's device in a way that could be tied to me. Other times, while some "leaps" would be involved, I could basically deduce myself that someone who had looked for information on some "X", and information on some "Y", might really be thinking about some "Z" that isn't easily arrived at from either X or Y in vacuo.
Spying, in the sense you suggest, can't be ruled out by the above. But, I would ask - why even spy? Is a company like "meta" really going to get much more useful (from their perspective) info by doing so? Particularly given the COST? It's becoming more realistic, arguably, but, really, these companies have had more than enough info on just about anyone for well over a decade to keep their algorithms and such well-occupied.
People gladly hand over tons of data constantly ... with full awareness and intentionality, and otherwise. The vast majority have no idea what statistical inference and other techniques can suggest based on seemingly obliquely connected info. Further, most users are so accustomed to "cookies" and other hidden types of tracking, and ignoring EULAs** ... really, it's hard for me to imagine a good case for doing anything more ... "invasive" and ... legally / otherwise dubious.
Edit: mostly came back to add one of my favorite (ab)uses of (statistical) inference:
https://youtu.be/Oseqh7SMIvo
(also, added the bit below about EULAs)
* Outweighing significantly, I'd suggest, other quirks of human perception, memory, etc. that may diminish awareness and recognition of potentially related events. I write "suggest" mainly because I don't have ready refs to offer this second and don't have time to dig a couple up ... IIRC, the research that exists strongly favors compounding, though, of course, this could be argued to be influenced itself by human psychology (including social and economic factors, e.g., "publish or perish" etc.).
** Jargon buried in legalese, what a genius way to get just about anyone to agree to just about anything! If only John, King of England (in 1215), had been more skilled in the ways of the EULA - perhaps "King Charles III" would be emperor of the world now. Oh utopia denied ... kek.