Also, don't forget that you don't belong to one community; it's not like you log in one day and get assigned to your immutable chunk of the internet. You wander around the read versions of various communities, pick a couple of communities you invest it, pick several more to half-follow, get tired of the abuse in one and switch to another, meet someone in real life who encourages you to join another community, and so on.

Basically, the sort of "tribalism" that you'd encounter is whatever "tribalism" you associate with HN, because the structure I describe already half-exists, scrounging around the remnants from reddit and Facebook in tens of thousands of forums and active blogs and other such things. I don't think it produces any more or less "tribalism" than you get from anything else.

Social media is like being in a room with thousands, possibly millions, of people. If you have a gathering of this many people in real life it's entirely possible for the crowds to go out of control and for them to turn into a mob. This mob then gets worse through one-upmanship.

Subdividing these groups might breed new tribalism, but it'll make it less likely for a situation to spiral out of control. It takes more time for information to pass the barrier between communities. You also won't get as many people engaging in group-think, because your in-group isn't as large.