I understand your point, but people are volunteering to give up this information. At some point, grandma has to learn to stop calling the psychic hotline.
How do you explain to grandma that publishing photos of her grandkids is to the detriment to society as a whole?
I think it's important to remember that Facebook provides a service people really want, which makes it hard to convince them to mistrust it.
Most countries have consumer protection laws to protect the rights of customers and stop deceptive business practices. When a significant portion of a company's customers feel deceived, it's easy to see that the company is not being honest to their customers.
[0] https://www.theatlantic.com/technology/archive/2012/03/readi...