Earlier quoted context omitted.
How are people being exploited? Where is the harm?
They get shown ads that might be more relevant to them than just an ad picked at random, which might make them more likely to buy the product displayed in the ad. I guess that could lead to some extra harm to their wallets, but otherwise I am with you on this one.
...but the product they're actually offering is "behaviour".
They sell access to the levers to influence behaviour.