For a while Google+ recommended content that your friends liked or interacted with. I once got a Google Play app recommendation that highlighted the review a friend of mine posted on it. I was TERRIFIED that it did that by default and spend the rest of the day going through ALL settings on ALL online services that allowed connecting with friends in any way. Also, you could go to my youtube profile and could publicly see what videos I liked. A friend asked me about it and I was mortified!
I’m gay and didn’t want people to know when I was younger. I think everybody who says they have nothing to hide has either not thought very deeply about what they may want to keep to themselves or does not understand the principle that people should only ever know about you what you want to share with them.
Also, if being an open book is the norm, everybody with good reasons to not be completely open (like I used to be) will eventually stand out from the crowd. Keeping everybody else’s private stuff private also means you can keep your own stuff private.
There’s a great quote from Snowden about the right to privacy you can look up here. Excerpt from the page:
"people saying they don’t care about rights to privacy because they ‘have nothing to hide’ are no different than people saying ‘I don’t care about freedom of speech because I have nothing to say’ "
Someone mentioned Firefox Focus.