If it said up-front "this data will be collated and eventually used to emotionally manipulate you", I'd agree with you that people give the information up of their own will.
Because I have yet to be convinced that knowing how open, agreeable, etc. someone translates into manipulation and the article did not even attempt to explain that, it just said this has some vague relation to Trump & Brexit and left us to assume it was a magic X factor.
"Mr. Trump’s digital team used [individually targeted] posts to serve different ads to different potential voters, aiming to push the exact right buttons. [...] A pro-gun voter whose Ocean score ranks him high on neuroticism could see storm clouds and a threat: The Democrat wants to take his guns away. A separate pro-gun voter deemed agreeable and introverted might see an ad emphasizing tradition and community values, a father and son hunting together."
I don't know if there is danger, but I find the idea very scary that there is an organization that collects very intimate data on me without my knowledge and uses that to manipulate me. They are not really trying to find points that people agree on with Trump. They are trying to create a strong emotional image that works with the person in question and connect it to Trump.
That's advertising in a nutshell, though. You can (and should) limit the information you put out. You can (and should) learn to recognize emotional manipulation of all kinds from all sources.
a) This degree of personalization is completely unheard of.
b) No one except the most paranoid expects that when they fill in a quiz that information will be recorded and stored.
c) Normally with personalized advertising (or advertising in general) the worst that can happen is that the person advertised to will buy something they don't need. In this case it can have implications for many other people.
A) Not really, Google knows way more about us.
B) That sounds like a problem with education.
C) This is by no means the only targeted political advertising. They have systems to track what you respond to and have done so for quite some time now. Calls, letters, even people visiting at the door don't happen by accident.
