The problem is not targeted marketing per se, but how it is used and by who. Traditional advertising is regulated to avoid falsehoods, exaggerations and exploitation of vulnerable audiences (eg. children). Social media combined with big data analysis allows for deliberately seeking out vulnerable target audiences and then feeding them any "alternative facts" you like to influence their views and, most importantly, is completely unregulated.
This is abhorrent enough when the marketer is a corporation trying to sell a commercial product, but is becomes truly worrying when the marketer is not trying to sell toothpaste or an iphone, but is instead aiming to influence political, cultural and social opinion for their own gain - this is manipulation of the lowest form.
And just a minor point as an example from which you can draw your own conclusions - Steve Bannon is a board member of Cambridge Analytica. If a government interrogating your data to feed you lies that play on your worst fears and steer your reactions would be (rightly) viewed as fascist, authoritarian and reprehensible, what about individual politicians now seated on the US National Security Council?.