Earlier quoted context omitted.
My theory is that it's because social media systems encourage and publicise agreement, while discouraging and hiding disagreement. This leads to a huge positive feedback effect on any meme that catches even a tiny number of people's attention, no matter how crazy. On twitter, you signify agreement by retweeting; this takes seconds and permanently adds to the tweet's score. But to disagree, I have to write a message o…
The problem with downvoting (at least simple versions) is that it allows separate communities to interfere which each others' communications. So, say you have hunters and animal-rights activists. They should be able to talk among themselves on Google+, Twitter, or Facebook without interference from the other side. You don't want one side to start downvoting the other side's posts into oblivion so they can't even talk…
I was using this very analogy to explain the advent of CCG "formats" (MTG, Hearthstone) a few days ago and it seems like the same analogy could be used here. There is a point where a discussion/dicussion room contains too much data to be manageable and the only way to fix that is to break it up into chunks.
We see it on Reddit, too, with subreddits becoming "too big" and naturally breaking up into smaller subreddits.
I suspect there's a universal truth hidden in there somewhere.