Hacker News is as much, if not more, of an echo chamber as any Twitter newsfeed is. At least on Twitter you can choose your echo chamber, and if you desire, even fill it with opinions from diametrically opposite sides of an issue. You can simultaneously follow @GoldmanSachs and #OccupyWallStreet, for instance. You have no choice on Hacker News, except to subscribe to an echo chamber generated by some nebulous algorithm subject to in determinant cognitive trickery.
The reason Hacker News and Twitter enjoy such high engagement rates is precisely because they are echo chambers. Groups tend to share opinions and tastes, so naturally enjoy the same clusters of reading material. I enjoy the echo chamber at HN, because the opinions of the group that composes it are important to me. I haven't been able to hook myself on Twitter, but I imagine it's because I haven't found the right echo chamber yet.
Twitter and Facebook have been pretty effective for Yes campaigners in the Scottish Independence referendum. It's taken the No Thanks folks by surprise by how effectively it's being used.
Social media can be a force for good under certain circumstances.
In addition to doing all that you describe, Twitter also encourages you to ignore anyone claiming otherwise - somebody has a difference of opinion? Block and unfollow!
I'm pretty anti-social-media in general, but Twitter is definitely among the worst.