People who buy ads against Facebook and Google profiles don't get to see specific info about the users they are targeting.
In this case the firm doing the targeting is also the firm that can see all the profile data.
That there's usually a middleman is something of a random detail. You don't look through millions of profiles and pick this ad for this person. You group people by common characteristics and then match ads to those. That a different advertising firm is going to suddenly do something so much scarier sounds more like science fiction than fact.
I agree with you but my first reading of the OP lead me to believe that she/he meant that Google and Facebook are advertising firms.
Admittedly, not as annoying as all the rest, but still, their business is advertising.
Google and Facebook both have a consumer-facing presence so you think of them as being more accountable, but they're both advertising businesses, no doubt.
It's different than Google's insight into your buying interests from Gmail, because they're the same organization.
People should know this intellectually at least, but they haven't been faced with the implications.
Analogy: I give a box of old family photos to a cousin. The cousin later needs medical treatment, and sells the photos to a pawn shop. Then the shop goes through the photos looking for the really outrageous ones to sell to third parties. I get really mad. Who do I blame?