ff.net has been in complete shambles for the past several years and is a notorious hive for plagiarism. The mods also nuked thousands of fics from the platform for reasons ranging from justified to incredibly questionable, and the site has had two of these mass purges in its lifetime. It's also been on the decline for years with constant outages, unresponsive mods, and a piss-poor tagging and search function. The owners of the site do not seem keen on even really giving the base code a spit shine, and I'd really question the security of it overall. I would not be surprised if they have a major data breach in the years to come. Don't get me wrong, I love it to death in a nostalgic way, but I can see why it's not exactly a great place to post fic these days.
Wattpad, usually the second site mentioned in these discussions, hosts tagged erotica alongside the rest of its content with no filter by default. This means you can find erotica in any search query you use unless you specifically exclude the tag. It's also a notorious hive for rpf, and allows rpf erotica. It also allows sexual content of 16 year olds, as it states in its code of conduct that, quote, "the age of consent is 16+ on Wattpad".
And really, those are the only notable contenders against ao3. Any other website you could list would fall into "niche" territory, and most writers do want their work to be seen so they can gain traction and potentially a clientele later on if they want to enter writing professionally. Plus, blogging sites like Tumblr tend to not be very conducive to posting fic as they aren't designed with a book format in mind, so word limits are tighter, you have to manually stitch a sort of chapter browser together by yourself as you post the fic, readers will encounter your fic in reverse chronological order, and honestly I could go on. Plus, Tumblr's tagging system is also complete garb, and if people don't reblog your fic, you're SOL. So, while I've heard of Fanfiction Online and have seen some promise in Quotev (I've made an account there recently, so consider that a "to be continued"), sadly, in terms of usability and popularity, all alternatives are overshadowed to the point of complete obscurity by the behemoth that is ao3.
Trust me, I hate that website and the cesspool it plays host to. Ever since I learned just how bad it was over there, I haven't spent a single second of my time on it. However, I can't blame aspiring writers for using it in the same way I can't blame other content creators for using YouTube rather than Dailymotion or Vimeo, despite YouTube's openly predatory practices towards their artists and child audiences (the Elsagate stuff never stopped, they're just using Mickey and Sonic now). The site holds a soft monopoly over the realm of fanfiction publication, and it's going to take a website that directly outclasses it in terms of user-friendliness to even really start giving it trouble and to convince the majority of the audience to switch over.
I'm all for taking as much of an audience away from that hellhole as possible though, so I'll gladly vouch for any alternative website that directly outclasses ao3 purely by design. Hell, ao3's code is completely open-source. If anyone manages to literally make a better ao3 (maybe even implementing those lofty promises of multimedia support that they've been harping on for years, hm?), I'll make an account on there the moment I hear of it and spread the word of it as far as I can manage.
Honestly, I think it really speaks to the state of fan culture overall that the best place to view and post fanfiction online is a csem hive, and I really want to change that message if I can. Unfortunately though, as much as I want a viable alternative, we're stuck between YouTube and Dailymotion, and I cannot blame anyone for wanting to stay away from Dailymotion.
Also @proship-blocklist since it was your post that was involved in this, feel free to add your input as an author/reader if you feel so inclined.