Allow users to specify more fine grained privacy options.
Questions to consider:
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I would say, yes. The reason ppl usually want to hide stuff is because they don’t want others to know about it. Not hiding their activity would defeat the purpose, imo. 
Lists, however, I feel are different: If a user creates a public list and adds a book they have hidden from their own page, that’s the user’s decision and fully within their control.
Yes. I think users need to be able to trust that if they say “hide the fact that I have read this” then the site will absolutely never surface anywhere in any view the fact that they have interacted with that book.
On adding a hidden item to a non private list: I think the site should prompt “are you sure?” about that before doing it, to avoid accidental leaks due to the user forgetting or not realizing that a list isn’t private.
I agree with the sentiment here, if you want something hidden in your profile there shouldn’t be a way for anyone else to discover it. Per item would be best for maximum flexibility, perhaps with a checkbox to auto-hide adult content for ease of use, though I appreciate how much work that would be.
If an item was added privately, would a user still be able to grade it so that it ends up with a score?
It does look like if you have everything private, you can still grade things and they are visible. Or at least if unchecking show my hidden activities (to myself) is to be believed. Honestly, I didn’t see any difference in what I was able to see when I turned that off.
It would be nice to have an opt in for sharing on each item. Even though it’s a fair amount of work when the vast majority of your collection is fine to share, I’d rather the security of not shared by default
I’m guessing that data can be anonymized to the public. So you should be able to grade, and instead of showing “X considered this harder than Y”, it would be “A Natively user considered this harder than Y”
