system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 Why is the breaking news about Facebook’s selling of user data such a big deal?

I am very confused. I was under the impression that there is no such thing as online privacy, that every click we make and page we view is tracked and bought and sold by various companies and entities. Furthermore, I thought that this fact was common knowledge. Datamining is nothing new. Targeted ads are nothing new. What is different about this story that has millions of people ready to burn down Facebook HQ?
gpt
What Cambridge Analytica is alleged to have done goes way, way being simply collecting information that people willingly gave out online. It was how they did it, how they analyzed it and what they did with the results.

If you were to take part in an online survey that asked you about your opinions on certain political issues, you wouldn't be surprised to find that this information was being collected to create an idea of what the current mood of the nation is. That's fine, although you might appreciate it if you were told that this information was being collected for this purpose, and you'd also hope that this information was anonymized.

But suppose you did a "Which *Lord of the Rings* character are you?" survey, asking questions like, "You're faced with a terrifying monster. Do you (a) run and hide; (b) call for help; (c) stand in front of it and yell, 'You shall not pass!'?"

Seems harmless, right. Just a bit of fun. Congratulations, you're GANDALF!

But if the allegations are true, it is not just a bit of fun. Although the questions seem totally unconnected with anything political, CA claims to be able to analyze it and build up a very detailed psychological profile of you: your political beliefs, your hopes and fears, your race, your income -- everything about you, even very private stuff that you have never told anyone. Their boast, in fact, is that by doing this, they can wind up knowing more about you than you know yourself.

In fact, they know you so well that they know what kind of propaganda you're most susceptible to, how you can be persuaded to change your mind, how you can be subtly manipulated without your being aware of it. Suddenly, whenever you go online, you see ads that you're not *consciously* interested in, but which are filtering through to your subconscious. These ads, by the way, are tailored to you personally.

A few weeks later you answer the door to an opinion pollster. They ask you a few questions and record your answers on a tablet. What you don't realize is that the questions they ask you are again carefully selected to manipulate you personally -- using data collected online, they know to an accuracy of a block or two just what people in different areas think and believe. In fact, you don't even have to have been online at all: it's enough that your neighbours have, and there's a high degree of probability that you're similar to them.

And to make this even more sinister, they're collecting information not just about you, but about your Facebook friends as well, even those who don't do any of these online "surveys". By looking at your friends, they can discover a great deal about you; and they can discover a great deal about your Facebook friends as well, simply going on the fact that they're friends with you.

Now imagine that all this information is being given to just one political party, but not any others. You have no intention of voting for that party at all, but this same party now knows the best way to trick you into changing your mind.

This is a *very* big deal. If the claims are even halfway true, CA has discovered a way to brainwash millions of people.

By the way, this

> we signed away all ownership of our data when we signed the user agreement

isn't true. It's one of those conspiracy theorist claims that make the rounds whenever a social media site changes its TOS. You grant Facebook a licence to *use* your data and publish it online, because if you didn't, Facebook would be breaking the law every time it published one of your status updates. However, you still own your content, and you still have certain rights. One of those is the right not to have your data passed on to third parties without your informed consent.