See, the main fanfic sites back in the day that most people utilized (that weren't fandom/community specific forums and such, anyway) were FF.net and Livejournal. FF was arguably the larger of the two just because it was a database for EVERYTHING, and with few exceptions (i.e. god help you if you were an Interview With A Vampire fan because then you had to fight with litigious as fuck Anne Rice), anyone could essentially post their screeds about anything pretty much. LJ was smaller but arguably had the better quality content since it could be moderated by members of the community and as such things like My Immortal-esque fics and trollfics could be weeded out, but this also sometimes made it harder to access the content if it was part of a closed community.
As years passed, FF.net's quality rapidly declined (much in the same way as deviantArt, in which it mostly became a haven for autistic fetishes rather than actual content), and the banhammer swing on posting explicit/R-rated material was a serious nail in its coffin. Around the same time, LiveJournal, due to complaints from the proto-SJWs like the ones seen above, did an event that was called the Great Strikeout - numerous communities, often fandom/fanfic related, often with explicit material, got banhammered as well.
These disgruntled fans then moved on to try and find someplace else to shelter and post their stories. Some went to Dreamwidth, which had been started as and was being advertised as "Livejournal 2.0." Many more moved on to Tumblr. And until AO3 surfaced out of the abyss, this was where the majority of them were posting.
As it is, LJ's remaining users are mostly old-school posters who survived the various exoduses, and Russians. However, since it is now completely owned by Russians, that means it falls under Russian censorship laws, which means no gay shit. As such, there's now a second exodus going on in which these old-school posters and Russians are now fleeing en masse to Dreamwidth to get out from under Putin's bootheel. Those that DW doesn't take on are probably going to end up swelling AO3's ranks. Those who are disgruntled with the Tumblr community turning into the same bunch of fucking gatekeepers that made LJ worse are going to fucking swell AO3's ranks.
