People have civil discourse in IM/IRC/SMS all the time and none of those media are seen as naturally aggressive.
People in real life don't monologue all that often and most communication is "short form".
People in real life don't monologue all that often and most communication is "short form".
Unlike Twitter, which incentivizes people carrying on infinite numbers of asynchronous conversations, and encourages people to find them by bringing up 'interesting' ones on the timeline.
In the heyday of blogs, long form posts weren't immune to those effects either and that's why blogs invented things like spam checks and comment lockdowns and comment moderation tools.
IRC - on specialist forums perhaps there is some civility because everyone in that forum is likely experienced and interested, so mutual respect and good manners reign.
But when you put together a heterogeneous group of people with no defined clear goal for the group and mix in anonymity by default, you get 4chan.
It'd make an interesting sociological study, which factors (privacy, anonymity, interest group, etc.) correlate with aggressive online social groups.