Honest question. I'm unsure what the danger is in letting these companies acquire data on us. We get a lot of benefit from using their products for free. Why should I care about giving my data to them as a cost of admission?
Are there personal things in your past you don't want your insurance company to know about? How about your employer? Or the IRS? Even your spouse? What if someone showed up one day and threatened to your your secrets unless you pay up?
Is that worth the price free access to your high-school friends' duckface selfies?
Once tabloid journalists start trying to infer embarrassing things about politicians based on metadata we'll probably have some progress.
Trivial example: Let's say sometime in the future you plan on running for political office, or you become a journalist writing a story on someone powerful or otherwise become someone who powerful people would like to discredit. Having a log of everything you've ever done online would be very useful in causing you a whole bunch of problems.