Some people have told me that they WANT companies knowing their preferences to create a "better web / inter webs experience"
--Alan Armstrong
Typically, people assume that if companies have lots of information about them, the Internet will be pretty-much the same as it is today except the ads will all be for things they like. Win-win!
But that's not how it works. It's not just the ads that will change. Reality as you perceive it will change. When friends post complaints about products, you will see less of those complaints on Facebook and Twitter, and you'll see more positive reviews, because that's what encourages you to click on ads and buy things.
What they know about you will affect what you see on social media like Facebook or Twitter, what news stories will appear when you go to a "newspaper" like torontostar.com, and what products you will find when you do a search on Google, within Amazon, or on eBay.