I'm going to write a follow up about how I wasn't mad about the feedback as much as I was mad nobody had told me sooner. We were a young company and didn't really have a management or mentorship skillset yet. We would all develop that together in the ensuing years and this was a major part of my motivation for pushing for it.
Sounds like there weren't any grownups around, honestly. I'm also getting the feeling that it's a fire-happy kind of place, maybe I'm wrong but that could explain the note-passing childishness. Maybe they were afraid of getting canned if they talked back to you.
I don't know if this is a lesson about being nice or a lesson about dealing with different personality types, I guess it depends on how much of an asshole you actually were. Personally I'd prefer an environment where you call my work "shit" and I say you have your head up your ass, it saves time and it's better than passive-aggressive weirdness and underhandedness but not everybody is like that.
Any place where people are arbitrarily fired is going to be toxic though, the reaction from the CTO and a few other things I've heard about Facebook kind of makes me think that's what's going on.
