It's definitely seems like one of the more apparent trends over the last five or so years (not that everything was sunshine and roses before that). As @Jacob1092 has mentioned, it appears to fall line with a more general 'tribalisation' of many Western societies, with a corresponding deterioration of discourse. What I don't know is how the interplay there is between the internet and society at large works, and which way the influence flows. Is the internet merely reflecting changes wider in society, or is it responsible for them?
The development of 'Web 2.0' more tightly integrated social media into the fabric of the web, and at the same time, the proliferation of smartphones, 3G networks, and general ease of access to the internet massively opened up internet use to more people at more times of day. Not only has the way we use the internet changed, but who used it and how often. For one thing, no more queuing up for Internet cafes.
And then the question of 'size' and how it relates to 'community'. @Ralizah mentioned this and I think it's really important. People's desire for debate is strongly tied up with a desire for community, and the desire for community is often comes with a desire for a sense of 'belonging'. Not necessarily in any overly soppy sense, but enough to feel that the people you're talking with are more than just apparitions. At the very least we want our words to make something of an imprint of the world (otherwise we wouldn't bother writing them), but writing to an ever changing, ill-defined audience is like trying to carve your thoughts into a running stream. Im order to feel like part of a community, you need to be able to somewhat picture it, but in 'wall-less' communities like Twitter, or even vaguely defined communities like 'fandoms', this is nigh on impossible. How can you feel like 'part of something' when you've got no feel for what the thing you're part of actually is - other than a stream of bickering voices? How can you turn each of those voices into a person when your only contact with them is a single sassy tweet in a stream of hundreds? Where does that leave the posts that aren't sassy, or perfectly delivered memes, or somehow outrageous enough to be worth interacting with? Suddenly any content that doesn't deliver the maximum amount of impact in the timeframe any given user is likely to engage with it will be forgotten. So there's a race to the extremes in order to stay relevant. Anything that doesn't contain the necessary amount of snark, sass, flamboyancy, or general oneupmanship isn't shared or retweeted, and anything that anything that isn't shared or retweeted gets quickly discarded. That's the basic food chain of social media on a sufficiently large scale.
If our social environment grows too big, we need to become caricatures of ourselves in order to stay noticed and relevant. We're more likely to cling onto existing trends and narratives in order to be more easily understood within the existing, accepted framework of ideas that surrounds a debate. We're more likely to make caricatures of other people in order to mock them and define ourselves against them. So I think 'scale' has a lot to do with it, as well any any/many more subtle points.
I'm not sure how much of which of the above points contribute to the current online social climate found in many fandoms and other communities, and I'm not even sure to what extent each point is true. But that's at least what I think at the moment; fandoms are simply too big an entity to allow the sort of fine grained discussions that deal with the (often quite personal) reasons why people are actually fans of the thing in the first place. Discussions of larger scales inevitably fall along certain lines, construct tenets, and establish accepted narratives, and this inevitably obscures the myriad of different reasons why each individual might like the thing and want to share their feelings.
Anyway, sorry for the ramble. There have been so many changes in the last few years that it's hard to tie their influence together very conscirelt (let alone accurately). But it's a really relevant and interesting topic, so thank you for starting the thread!