Almost all web sites are visited for some transactional purpose or in response to some specific trigger. But some web sites (Twitter, Reddit, Facebook) are their own trigger: if someone’s bored and has a browser open, they may end up at one of those sites automatically, without any exogenous trigger.
Conversations can arise there with no apparent cause, and they can naturally continue over days or weeks as participants naturally return to their “water cooler,” or as more new people stumble into the conversation.
A related notion: Twitter is a “third place.”
==TODO: I don’t think this characterization is very precise. I think there are several things going on here, and by not distinguishing them I’m not being clear about the differences between e.g. Twitter and Facebook. A few key components: these are places where people spend time by default; conversations are semi-open in audience and time; algorithms can surface conversations to potential participants…==