Out of curiosity, what happened to Pinterest? I was never an active user, and the only thing I observed was the mainstream interest in it fall, so it got out of spotlight, but i dont remember anything bad actually happening that triggered it.
A lot like Quora, I think a lot of people see their tactics as super hostile to users. Things like bait and switching to force you to sign up to see the content. Tons of spam on the site. I know a lot of people hate that they have just dumped so much trash into Google image results, again, forcing you to signup when you actually want to see the content. A lot of dark UX patterns like those seem to be the biggest complaint I've personally seen and experienced.
