You probably don't talk to the shop assistant in produce about your medical problems and then ask the deli staff about life insurance.
You probably don't book a four night away break in produce and then meet a friend (who happens to be a divorce lawyer) in the deli, where the staff know all about what he does for a living.
You probably don't discuss homosexual relationships in produce and then call your boss, Colonel Gaybasher, from the deli's payphone.
One of the most valuable things about privacy is that it lets us compartmentalise our lives. That helps people we know by not inflicting parts of our lives they don't care about on them. It also helps us, by allowing us to explore and develop one aspect of our lives with people we trust to help us, without allowing others who we don't trust to know about it or interfere with the rest of our lives as a result. One specific example of the latter idea is that maintaining privacy means someone who is trying to assess us doesn't get incomplete information that they consider relevant and then jump to conclusions based on getting the wrong idea.
The likes of Google and Facebook are now so ubiquitous that the complete destruction of that privacy for millions of people is a profit-seeking executive's sneeze away from happening. It's bad enough that they rely on this sort of perma-snooping on their own users on their own sites for their own advertising. It's downright creepy that they are abusing the nature of the Internet and the ignorance of most people about that nature to spy on people visiting other sites as well.