I disagree with Tumblr and Reddit being compared with Facebook in this regard, and completley disagree that HN is in anyway similar in this regard. Facebook will automatically stop showing you things you disagree with, whilst it has to be a conscious choice on your part on Reddit and Facebook.

On HN you don't actually get to choose the content in any meaningful way, and therefore have no means of creating an echo chamber for yourself through other means then down voting.