Whenever I discuss this topic people seem to almost take pride in giving their information to Google, Facebook and the thousands of other companies they have no clue about.
I think it's cognitive dissonance rearing its ugly head again: those people have tied their identity at some level with the use of the products and an attack on e.g. Google becomes an attack on them.
The concept of fairness also plays a role. Most people feel that Google should get something for providing their services and they also feel that it's ok if they are shown some ads. Fair deal. What they have no idea about is how much data is extracted from them, who it's shared with and for what purposes it's used. But we can be almost certain the answers to those questions is more and more data, more and more parties and more and more purposes, because data is forever.
And yes, real harm can come to you if you are promiscuous with your information. Additional information should be accessible through your favourite search engine. Hint: home invasion.