There's an adage that says "If you're not paying for the service, you're not the customer, you're the product."
There's nothing but truth there. As you're probably aware, Facebook, Google, and most other modern internet companies make their money off of the ads. Ads are such a part of life that they're easy to forget about and they've been around forever, over a hundred years in newspapers, on TV, even at the beginnings of movies. No big deal, right?
What makes Facebook and Google's ad space so valuable isn't that you're going to see them every time you open them (though that's part of it), it's that they collect so much data on people that the ads they sell can be hyper-targeted. If a company wants to, say, spread disinformation about the 2020 Presidential Election to people in swing districts between the ages of 20-50 who are white, make between $15,000 - $20,000 per year and are also die-hard Q-Anon believers... they can. And do! This kind of targeting can be extremely valuable to a lot of different people.
And then there's the data brokers who gather up data from dozens of different sources, aggregate it all into one place, and then sell it to the highest bidder. It's a billion dollar industry ($19 billion by US corporations in 2019 alone) and although some of the information isn't always accurate, it's often good enough to find someone with some pretty basic information. If you want an example, open a brand new Incognito window (ctrl+shift+N on PCs) and do a search in Google for anyone's first and last name and the word "address". If it's a common name, put in the city. Chances are good you can find their address without issue.
And the current situation is only slated to get worse as "smart" objects are increasingly available and they'll be collecting a whole new slate of data that we haven't even thought of yet. Everything from smart fridges keeping track of what and how often you eat, to smart washers monitoring when, how, and how often you wash your clothes, to other smart devices that haven't been invented yet. All of that data being collected is almost certainly being aggregated and sold because there's a lot of money in doing so.
So, unfortunately, it's pretty much all about the money. But how bad can all this data floating around actually be?