it's interesting that some people are on the internet but is very well insulated! AO3 is very well known for me...
Not sure I'd call it "insulated", the internet is just very, very vast, even when considering "just" the English-speaking web. Then you have all the other "versions" out there too that are kind of hidden to most people :)
Anecdotal, but also first time I heard about AO3, and I'd consider myself having broad interests and generally well-read, although my interests doesn't include fanfiction so maybe not so weird I haven't heard about it before.
FWIW the vast majority of writing on there is decidedly mediocre. There is also an even more inferior alternative called Wattpad.
Funnily enough you learn that in general we aren't all that different in our tastes, it's just that what men like to watch, women like to read / imagine.
Edit: to paint the picture, this[0] was sent to me a while back :-)
[0]https://www.tiktok.com/@alexarowe11/video/746846214634761757...
I remember when I first stumbled across the main Antimemetics Division storyline on SCP. https://scp-wiki.wikidot.com/antimemetics-division-hub