My college-era experience with the internet (2002-2006) is heavily based around blogging. Once blogger, xanga, livejournal are a thing, I never look at geocities again.

MySpace and Facebook both take off during this time, initially targeting different audiences.
It's hard to even explain what the internet was before social media if you weren't there, because now social media IS the internet, and by extension social media IS fandom. This was not always the case.
I'm not going to get too deep into fandom history specifically, both because I came into fandom late (at least as an active participant, vs. simply a visitor to fansites), and because so many others have already done that.
If you want to learn about zines and fansites and Yahoo! groups and the LJ years and Racefail and Strikethrough and the creation of dreamwidth and AO3, there are places you can learn about that from folks who were there. http://fanlore.org  is one place to start.
So we come to the tumblr era of fandom. The idea that tumblr also drastically changed fandom is hardly a hot take at this point, and I don't think the idea that the lack of control over one's content is a big part of this shift is a hot take either.
The change in how the search box worked and the deterioration of tag-browsing as a feature people actually used is definitively a factor. (Though, as I've pointed out before, it IS a feature that still exists, something a lot of people are still confused about.)
And you could definitely pinpoint the changes to the tumblr search box as a major turning point.

But so was the reblog itself.

The reblog feature was native to tumblr when it launched in 2007, and it precedes the retweet feature by 2 years.
The reblog feature ensures that no tumblr user will ever have complete control over their own content. Once you make a post, there is nothing you can do to stop it from being taken out of your control, out of its original context, and away from its intended audience.
I would characterize this general loss of content control, and the degree to which we've come to accept it as normal, as a major shift in the landscape of the internet. As major as the Web 2.0 shift.
And that loss of control by creators is mirrored in a loss of control by consumers of content. Of course, in a social media based internet, most of us are both.
And we all know this. We all complain about ads, sponsored posts, bad recommendeds, the inability to avoid users we've blocked. We all know how easily a successful post becomes an out-of-control post we wish we'd never made.

We know exactly how much the loss of control sucks.
There's a double-edged sword here, of course. Fandom veterans will talk about how fanfiction used to be much more unwarned and untagged, and you took a much bigger risk opening that content, even as you had to do more work to go find it at all.
And if fandom consolidating onto social media platforms also pushed us to a culture of tagging and warning, then arguably that's some good that came out of it. Of course, corporate-owned platforms will never be good for fandom in the long run and that's why AO3 is so important.
AO3 tries to give us the best of both worlds for fanworks: you can find what you want, and also avoid what you don't want (including ads and having your data sold to third parties).
Nope, AO3's not perfect. But the state of fanfiction in our social media driven internet age WITHOUT an archive like AO3 would be a nightmare.

I mean, more of a nightmare than it already is. Seriously.
The point of me recounting all of this is to try and illustrate how much of the current hostility over fan content probably stems from that loss of content control. The toxicity of the purity discourse has made it hard for some of us to look for the root cause.
As Sula says: https://twitter.com/SulaSafeRoom/status/1200212536517369857
It's understandable that when faced with harassment, threats, suicide-baiting, doxxing, and other completely unacceptable behavior, people tend to band together and concede nothing.

I don't blame people who've been the targets of that harassment for not wanting to engage.
I'm engaging because I want to see things change.

And I want to start by saying, you're right about this: the internet today is not the internet that existed when I was 15. The internet today probably IS hurting younger people in ways I don't fully understand.
https://twitter.com/outliersgeorg/status/1200203628633440257
I don't believe that an AO3 purge is the answer (even if there was a chance that would happen, which there isn't). I don't believe harassing people over characters and ships is the answer.
I don't believe in a 1:1 correlation between fictional tastes (in creation or consumption) and real desires, character, integrity.

And I do believe that there are SOME people arguing in 100% bad faith, hijacking the language of social justice for old-fashioned ship warring.
What I am willing to acknowledge is that underlying all of this, there IS a real problem.

It's not a problem I can solve by liking the correct characters.

It's also not a problem I can solve by yelling at the kids to get off my lawn.
It's a problem that's been overshadowed by a lot of very bad behavior by people of all ages, but underneath all that, it's real.

And it would benefit all of us to pay attention to it, and what it means, because it's much bigger than fandom.
If you made it this far in this very long thread, thank you!

Good faith replies are welcome.
In another recent thread I discussed various platforms where fandom is this days, and you'll notice there's usually a direct trade-off between control over content, and visibility. That's kind of where we are now, with the notable exception of AO3. https://twitter.com/apocalypse_anne/status/1199043063882141696?s=19
If we're going to have any hope of improving fandom culture, I think we have to look at prioritizing control of our content--for the wellbeing of both the creators & consumers of fanworks.

That's going to mean pursuing alternatives to corporate-owned social media where we can.
You can follow @apocalypse_anne.
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