i do think that a lot of this is just the result of a lack of lurk moar attitude in fandom/the internet in general.
when i was a tween who first found fandom in the late 90s/early 2000s, people didn't explicitly teach me how to interact with fandom. i lurked for a solid year before i signed up for my own account on the forum i'd found. (i can still remember how the adrenaline coursed through me as i signed up for my own account--i felt tingy and more than a little ill!)
by that time, i had a very good sense of social norms there. i still made a few mistakes, and the more established members smacked me down in a matter-of-fact but not unkind way. but i'd learned by watching. hell, by the time i started actively participating, i knew all the inside jokes!
as op mentioned, i don't think that people lurk anymore, and my theory is that the rise of social media/web 2.0 created a different approach to web communities.
today, every site is presumed to be for every person. the entire point of the really big social media sites is that everyone is on them. (this is one of the things i hate about them btw because it results in context collapse. i do not want to talk to my third-grade teacher, my favorite cousin, complete strangers, and my fandom friends in the same voice, but that's another issue).
whereas in web 1.0, the internet was riddled with niche sites/communities. you had to go out and find your place (and sometimes it took a while!). once you found it, you were invested in becoming a part of that specific community, so you did the research (lurking) to find out how people interacted, what all the unspoken norms were. by the time you picked your handle and made your account, you just knew stuff.
i'm sure this was not true of everyone, but it was true of far more people at the time. people looked before they leapt.
there are many, many reasons that i think that fandom has suffered from the web 2.0 environment. the fact that creators/writers/actors and fans are all on the same sites using the same tags for general publicity and for fannish nonsense is a huge problem. the way that sites are so big that people feel that their contributions (as with kudos above) don't matter is a direct result of the way social media undermines community and makes everything a performance of whatever your late-capitalist brand is. the fast pace of those sites makes people think that interacting with older posts is a bad idea. the lack of filters of the kind that we had on livejournal where you could determine who saw what or even just the way that forums often made you join before you could see content created walls within which communities could grow (think frost and walls making good neighbors).
i know we can't go back to the assumptions that operated before social media. we have to explore other options. i love when people make psas here telling people about fandom norms and history! i think it's the best thing! and maybe at this point that is the only way to handle it.
tumblr and ao3 are very weird sites in that they straddle the web 1.0 and web 2.0 kinds of internet.
from web 1.0 they get the lack of algorithms, the way you have to make choices about what you see, chronological arrangements, and (on ao3) lack of ads, etc. tumblr has a slightly slower pace than most social media; ao3 has a much slower one.
from web 2.0, though, you get scale, centralization (which is both ao3's greatest strength and greatest weakness), and the fact that it takes little effort to locate these sites--anyone, no matter their level of investment in fandom, can just stumble on them.
so you end up having a lot of people who are not actually fannishly inclined (aren't invested in a gift economy, don't really understand that fandom is supposed to be fun, don't really get the creative urge etc.) interacting with people who are fannishly inclined, and it causes some really problems. especially with younger people whose experience of the internet is as a venue to signify and perform certain kinds of morality/coolness/trendiness that are at odds with what fandom has always been about. basically: you have a bunch of normies clashing with a bunch of nerds. (obviously the normie/nerd divide is a spectrum and not a binary, so i'm overstating, but still.)
when you have people who are coming to fandom from different angles--some people who are coming to it as a provider of content just like all other media in their lives, especially elsewhere online; some people who are coming to it as a participatory hobby wherein we build community around shared affection for [thing]--there's going to be lots of clashes and weirdness.
i kind of think that fans need to go back to create set-apart spaces for fandom to happen. note that i am NOT talking about gatekeeping. everyone who treats others with respect would be welcome. but just having fenced-off areas that are explicitly for certain kinds of fandom interactions. where we can basically have our party away from the normies, but other nerds who are younger or just getting in touch with their nerdiness can find us.
i'm not sure how we'd go about doing it. but i think smaller, more intimate internet spaces are really necessary for fandom to be enjoyable. for fandom to be fandom tbh.
