Historically, many factors have worked in favor of online privacy protections. Although it was technically possible to build something monstrous with our data, it was difficult, time-consuming, and unlikely to be very effective or actionable. Many of these protections are now eroding.
Our US lawmakers are failing us. Europe has far tighter controls on what can be done with our data, including the "right to be forgotten". The newest legislation that recently passed the senate is particularly egregious, because it makes these new data collection regulations permanent, and cannot be undone.
As the field of data science continues to explode, the ability to analyze data is growing by leaps and bounds. It's like the difference between and abacus and a calculator; yes, you can do math on the abacus, but you can do math on math on math with the calculator, in a fraction of the time.
But, the expression "garbage in, garbage out" still applies. If your data doesn't really represent *you*, then you can still escape this.
Company A has data, and Company B has data. But A couldn't share with B, so the amount of influence that any single organization could effect was limited. Now A buys B and owns all of its data, and they can more easily share with Companies C, D, and E as well. The potential impact is enormous.
In the old days, everyone saw the same web page. Then news & content sites started making customizations, feeding you what they think you'll click on; the more of that you see, the more you click, creating a self-reinforcing cycle. But now, data from sites across the web is being combined, quietly creating a giant filter bubble that affects us everywhere. Now it's very possible to feel like "everyone" agrees with us, because we see things that are specifically chosen to agree with us, everywhere.
"Kids these days" - we are sharing more and more information online, without thinking that it's problematic. We're trained to share. As a result, there's more information about us, in more places, than ever before.
Now, the entire marketing stack is designed and sold end-to-end, facilitating the application of these technologies. Any marketer worth his or her salt can activate the remarketing technologies that bring all of this data together with the click of a button.