They make up the loudest majority, though. It's like stereotypes; the only reason a stereotype exists is because a majority of people share similar, outstanding traits.
You don't seem to understand how 'loudest' portions work.
There was some image about a square and a circle. Where only a small portion of the square and a small portion of the circle fight, but because the fighting is much louder than all the peaceful stuff, people don't see the peaceful majority that makes up the square and the circle.
Neither 4chan or tumblr are bad communities. There's just bad people trying to represent them.