This came up recently in my life, with a person who seems blissfully ignorant of any concern involving privacy.
For an obvious example, I said that putting up public notices (i.e. on Facebook) about your schedule was a bad idea. When asked why, my answer was that hundreds of people would be able to find you or tell others where you are. Try as I might, I have not been able to properly explain why that is a bad thing.
The topic is more general than that, including data privacy like photos, email, "home movies," or other personal information. On the internet, every company wants to be your friend, but their motives are never positive or even neutral.
So how do you convince someone of the basic idea that the vast majority of your personal information should be kept on a need to know basis?