They get the ability to target ads against people who match some characteristics. Facebook's business is literally built on advertisers inability to access that data. If they had access, advertisers would advertise elsewhere and Facebook wouldn't get a cut.
So while protection of privacy is always also the right of one individual, I think it is crucial to realize that this kind of power over "the masses" isn't something any free society could afford in the long term on a systemic level. If we look at a democracy with it's separation of powers, it's institutions etc as a delicately balanced electronic circuit — which has the ability to self correct — players like Facebook are like the introduction of new feedback paths that work better for people who don't mind to sell their soul when abusing them. Of course such new connections will have an effect on the whole system, but the crucial point is, that nobody really considered what this effect might be and whether what we have and like in democracy can actually survive this.
You cannot have free elections if the people voting in it are put into their own matrix-esque simulation of reality. How could we ever find agreeable consent if our realities have no common overlap by design?
I think a lot of people in tech sphere don't really want to see this as they are profiting from this. It makes us powerful wizards who see more and understand more than others.
Here's the media championing it:
"How a dream team of engineers from Facebook, Twitter, and Google built the software that drove Barack Obama's reelection"
https://www.theatlantic.com/technology/archive/2012/11/when-...
"How President Obama’s campaign used big data to rally individual voters."
https://www.technologyreview.com/2012/12/19/114510/how-obama...
"Obama, Facebook and the power of friendship: the 2012 data election"
https://www.theguardian.com/world/2012/feb/17/obama-digital-...
"the firm never tried to stop them when they realized what was doing, and even told them they'd made a special exception for them"
https://www.dailymail.co.uk/news/article-5520303/Obama-campa...
"Why are politicians now freaking out about a feature that has been publicly documented since its inception and that was discontinued three years ago?"
https://reason.com/2018/03/23/cambridge-analytics-dust-up-re...
Not spreading the boogeyman of 'they sell your data' will improve discourse on how to actually change things.
https://knowledge.wharton.upenn.edu/article/is-zyngas-depend...