Imagine, you share your observations with, say, a PI who was hired by a paranoid spouse or by someone's employer.
Facebook is like a vast and far reaching network of nosy neighbours of whom you do not know who they are chatty with. Maybe someone who likes to jump to conclusions?
I would think, that advertising in the local press or, having a sign outside the window to the effect that you were able to sell personal and assumed to be private information about the people who lived in the street would provoke an unpleasant reaction.
Doing something purely for your own benefit that you know will cause significant distress to others is sociopathy I think. Perhaps 'unethical' is the wrong word. 'antisocial' definitely covers it though it might not be strong enough.
Would you say there is absolutely nothing wrong with this?
The expectation of privacy. The window analogy may seem very clear to you, but a lot of people see Facebook as a sort of postal service carrying their enveloped messages to their friends, and they have to trawl through a pretty large document of legalese to find out how it isn't. Facebook is more like one-way mirrored glass.
You may not violate a freedom but the bigger question to me is why would you even contemplate the thoughts to infer, etc.
What is "wrong" is that people think it is ok to talk about others.
Next time you get together in a group take a poll on how many conversations are just talking about people.
That just shifts the question to how does it violate their freedom? That is not at all obvious to me. (NOTE: this does not mean that I'm saying it is OK...just that I don't see how it is a freedom violation).
My computer desk at home is near a large window, which I often look out while I am using the computer. From this window I can see people from my street walking dogs. I can see kids going to and from school. I see cars coming and going. My street is a dead end street about 1 km long, and I'm about 200 meters in. The street bends a little way past my place, so the last 700 meters or so are not visible from my place.
By casually observing people walk by, I've gathered data to make several inferences about people who live beyond the bend. I've figured out when some people are having house guests (by seeing people I've never seen before walking dogs that I recognize).
I've inferred sibling relationships among some of the children who walk by (by noticing dress and equipment patterns that clearly indicate that the same person is shopping for both).
I've figured out what kind of cars the parents of the some of the children drive (by seeing those cars stop when passing the children on the way home, and the children getting in, or seeing a strong correlation between days when particular children who walked by to school in the morning do not walk by in the afternoon and days when particular cars drive by in the afternoon).
Am I violating these people's freedom by making these inferences from what I see out my window?
If not, what is the fundamental difference between what I'm doing by observing people that walk or drive by on my street and what Facebook does by observing what its users do on its site?