Google knows a lot about its users. Facebook knows a lot about its users. FitBit knows a lot about its users. And so on.

But what happens when these companies all sell their data sets to one another? It'd be pretty trivial to link even anonymized users from set to set by looking for specific features. If I went for a run, Google tracked my location, FitBit tracked my heart rate, and Facebook tracked my status about my new best mile time, for example. Thus, Google can narrow down who I am in the other sets using pre-existing information that coincides with theirs. With enough overlap they can figure out exactly who I am fairly easily. Furthermore, each additional layer of data makes this discovery process from new data sets even easier, as it gives more opportunities to confirm or rule out concurrent info. So then when, say, Credit Karma, Comcast, and Amazon's data enter the fray, my online identity stops looking like an individual egg in each different basket but a whole lot of eggs in all in one. And they can do this across millions/billions of users--not just me!

I don't know for certain that this is a thing that happens, but... I have to assume it definitely is happening, right? How could it not? With how valuable data is and how loose protections are, this seems like a logical and potentially very lucrative step.

Right now, is there an aggregate version of "me" that exists in a data store somewhere that is a more comprehensive and accurate picture than my own self-image? After all, my memory and perception are imperfect and biased, but data stores aren't.

It's hard to get a straight answer on this because there are allegations of shilling everywhere when it comes to VPNs (particularly when you discuss specific providers). There's also this post which gets linked pretty frequently and which seems to throw a wrench in the whole idea.

For context, I ask because I have two main concerns:

1. I have been the subject of a mild internet stalking/doxing, and I have no wish to relive that experience.
2. I live in the United States and, if I am understanding things correctly, my ISP now has the right to sell my browsing data without my consent.

I have no love for my ISP and am all about the idea of blocking them from gathering data about me, but it seems the only other option is to hand all of my data over to another company who simply promises not to do anything with it. While I'm sure some of them are legitimate, how can you tell the difference between a genuine privacy tool and a honeypot?

Here is what just happened to me:
Firefox installed two addons - fxmonitor@mozilla.org.xpi and telemetry-coverage-bug1487578@mozilla.org into my browser silently, even though I've explicitly turned all the telemetry off.
This have happened before, and Mozilla apologized for it, however it seems that they learned nothing and are willing to do so again.
There goes the last scrap of my trust into Firefox. I suggest you check your browsers too.

Almost every content provider online tries to access some of your personal info, whether it's to keep itself afloat, improve functionality, or create profits. In 2014, Google made [89.4%] (https://revenuesandprofits.com/how-google-makes-money/) of its profits from advertising, all of which attempts to target users with their interests (though Google does allow this to be disabled).

What do you do to try and protect yourself from data collection? What software, programs, or browser extensions do you trust to protect you, and not just also monitor your activities?

If you don't do any of this, why not? To what extent do you think companies should be allowed to use your data?