I made a huge list of tons of single web fiction sites I could find about a year ago. I have only posted on a few though, and I have experience with a couple others:
(Oops I forgot to answer the questions in order, sorry in advance about making it annoying for your data gathering)
Tapas
* Pros: Nice reader interface, lots of interaction, good community, site staff who actually care, an app
* Cons: The text editor for prose is quite bad and lacks most formatting options; the audience for prose is quite small; the library system isn't very robust for archival readers and focuses too much on serial reading (I lose my place in stories a lot)
Tapas is better for comics, for sure. But I think its novels will improve a lot in the next few years.
* I heard about the platform from @KRWright when I made a rant post about leaving Wattpad this year lol. I knew the site from back in the Tapastic days and read some comics, and I subscribed to some comics via RSS, but I had no idea there were novels on the site until 2020. (Thank you)
Wattpad
* Pros: The most popular web fiction site by a mile. For prose alone it is bigger than all other web fiction sites combined. If you have a hit here, you can get a movie deal and make an entire career.
* Cons: The website is absolutely terrible for readers and writers alike, and unless you have a huge hit, having a story on Wattpad makes publishers LESS likely to be interested in you due to its reputation. Stay away unless you are already very popular on social media and can game the system. Bad formatting; deletes in-line links; no community forums; broken tagging systems; No post scheduling?!??!
* I have no idea where I heard about the site, but I started posting there in mid-2017 kind of as an experiment, then began posting more "real" stuff as I tried to break through the algorithms.
Archive of Our Own
* Pros: Robust tagging system; very good control over your stories and layouts; very easy experience for readers
* Cons: It's mainly for fan fiction, so original fiction is unpopular; very small readership overall even for fan fiction; not as "web 2.0" as other sites; no community element at all so it feels a bit "empty"
* I discovered this site back when it was in beta, as a successor to fanfiction.net. It still has lower readership than that site, but I use them both when I write fan fiction.
Royal Road
* Pros: Largest readership on average by far; active engagement for stories that fit its LitRPG/anime-esque niche; certain publishers look more favorable on this site than most others; extremely good text editor and analytics; the admins are very helpful and actively engage with the community
* Cons: EXTREMELY toxic readerships emerge if your story gets popular (though this is changing due to authors banding together to fight them); stories not in its niche will have much less engagement; a large portion of success is determined by conquering its convoluted Trending algorithm
Royal Road, like Tapas, may be a much better site for web novels in the near future, but for now it has a major problem with a terrible community of bully readers. If you have a thin skin about criticism it will be tough. Stories with gay men (specifically men) will constantly receive ratings bombs from anonymous jerks.
* I discovered this site in late 2018 when I began engaging the community on Web Fiction Guide (RIP) more often.
Scribble Hub
* Pros: Very good posting interface, possibly better than Royal Road's; allows more adult content than Royal Road (if that is the kind of story you want); very LGBTQ+ friendly
* Cons: Smaller audience (Tapas-sized); very low reader engagement; the forums are (currently) very toxic; the site is filled with adult content so it can sometimes feel sleazy
* I discovered this site as a word-of-mouth alternative to Royal Road; it's basically that site but more adult content and very few commenters.
Webnovels.com
* Pros: Has an app
* Cons: Please don't post your story on this site, whatever you do
+ I heard about this in 2019 when looking for other web fiction sites, mentioned as a site never to post on even the first time I learned about it.
Other sites I'm not as familiar with or are too small of readerships to really talk about. All web fiction sites now have pretty big drawbacks but I think Tapas and Royal Road have the most potential; the former because the staff seems actively engaged in improving itself, and the latter because there is an active effort to fight the toxicity on all levels. Still, neither of them are "huge" yet and it will take some time before web fiction really reaches a similar level to web comics.