I agree with your observation that a lot of privacy depends simply on social customs, and I think that in an advanced society this will always be primary way of keeping it.

I disagree with "and have had less [privacy] every generation". I think for the last hundred or so years we've been having unprecedented amounts of privacy thanks to urbanization and the anonymity that stems from it. In villages, where most people lived throughout history, there is no such thing as privacy.

In a way that makes sense - per-capita. But since villages were so tiny, the total number of people who knew your secrets was very small. Today, it numbers in the thousands/millions.

In this way we are sort of like a global village - gossip about you can and sometimes does travel around the entire globe. On the other hand, all those people "knowing" is almost as if they didn't knew anything at all - every one of those thousands/millions people have secrets of another thousands/millions to pick from.

I guess the primary place where our times are different is when it's you vs. a faceless Organization - a company or a government, where they can use your data against you while you have no target to retaliate against. In this way you may also say that we have less of that privacy with each generation, but that's because Organizations are quite recent things - your average villager of the past had neither need nor way to interact with their government the way we do today, and companies like we have now didn't even exist.

That last paragraph is probably the first time I thought that particular thought explicitly. At this point I'm not sure whether it's me conceding the point of continued loss of privacy in meaningful areas of life, or observing a strong correlation between diminishing privacy and progress of society, that may or may not turn out to be a causation of the "privacy vs. progress of mankind - pick one" form.

I'm going to book-mark your observation. Its concise and I think it will be reusable in many contexts!