I disagree. I believe that it’s more common for “private” to mean “between you and the company”. Eg. Private accounts on other services, or private information associated with your account.

You certainly have a point about "private". As you say, with a service provider. Or with family, friends and associates.

But none of that is relevant for "private diary". There, it's privacy between you and your diary.

Plenty of people had “private” LiveJournal accounts back in the day with no expectation that it was a zero-knowledge system. Just that they weren’t sharing it. I may not be a good indication of “most people”, but I just don’t think that most people think of privacy in the way that we (folks who know details of encryption and metadata leaks) do.

Fair enough.

But that's the problem, isn't it? I mean, many people do (sort of magically) think that their online stuff is private. And then they get pwned in one way or another.