For one example, we saw with Cambridge Analytica that political advertising can be tuned to your personal hot button issues. Those worried by immigration can be sent an outright (and outrageous) lie that remaining in the EU will result in all of Turkey moving to the UK. Those with different interests/concerns got different ads.
Turns out many of them were illegal too. No one knew until too late because it was invisible to all except the advertiser, Facebook (in this case) and recipient.
Until it becomes visible - it's really hard to make people care, or realise. Perhaps a few on the receiving end of the brexit/CA issue realised, if they remember the ads they saw, and read one of the many follow up stories. Like anything algorithmic, unless you a-b test how do you know what you saw that others didn't, what you aren't seeing, or where it discriminates?
Keep this up and we could easily lose most of the progress of the 20th century, invisibly.