When you post on reddit, instagram etc. you're giving the company your valuable data. (And it often is valuable, you put your thoughts into it and all that). Consider if you take 15 minutes to create value content, you'd get paid $5 or sth if you were doing it commercially. Now all that value is donated to big tech companies. (that's the only reason why they have value at all.)
So what's the problem with that? The problem is that reddit and others can instantaneously put your years-long effort behind a paywall. I'm not joking. Look at these:
when i want to see content on reddit (for research) or instagram (for local community) they don't allow me to see the data that users have contributed. Reddit claims that it blocks VPNs for "security reasons", meanwhile i'm using VPNs in browser's incognito mode to not be tracked. Instagram wants you to sign in sothat they amass more users and can take leverage of the network effect even more.
I can understand restricting who can post to limit bot activity, but reading must always be possible. After all, it's our data. That's exactly what happens when you give your data to big companies to "communicate with other people": your data becomes a source of income for them, at our expense.
I think you are overestimating the capacity and will of advertisers to actually identify people like that. The data may be available for it in theory, but it's unlikely they built the systems to make it happen because they don't really have much economical interest to do so. They care about market segments, not individuals.
I had to work a bit on this kind of things. Most of those systems are really shitty and ineffective.
The little advertisers aren’t the scary ones. It’s Meta, Alphabet and Amazon that are scary and absolutely do fingerprint in this way. That said, the scale of these systems really does roll right past the individual user, unless someone really cares about YOU, but unless you’re doing something wrong, they don’t really give a shit. CSAM, illicit substances, amd more could probably be pretty easily rooted out by these systems, but that wouldn’t make-line-go-up so it doesn’t happen.