> If you talk to your friends in person in a public space, would you be comfortable with restauranteurs stealthily following you around and analyzing your conversations? Why is social media any different?

Exactly, there is no expectation of privacy in a public place, so your friends should go to a private place if they want a private conversation (and similarly not have public social media profiles), not talking within earshot of others and expecting privacy, that people would close their ears while one talks. Restaurants, stores, and other such areas are explicitly not private places, I can't tell you the number of times I can hear embarrassing stories with no extra effort in my part, simply because people talk loud enough to hear.