How useful is online publishing? More specifically, will online platforms help you to become a (published) writer, help you to improve your writing, or simply help you find readers?
I have my doubts...
Answer me this: why are you writing, and what do you want to accomplish?
If you want to become a published author, and actually make a living by writing, the 'free to read' platforms might be the wrong choice. Because they are free to read, so how would you make money on them publishing your story then?
But, perhaps you've not reached that stage yet (I certainly have not 😅) so all you want to do right now is improve your writing skills, and get feedback on your writing.
The second reason you might want to use these platforms is to build up a name, to attract hundreds, if not thousands of readers, who will recognize your name because they liked your story, and then hopefully one day will buy one of your books.
The next part may sound a little negative, but I think it is only fair to be as honest as possible.
Several issues come up when looking for feedback.
One. Most active participants in fora (such as the former Wattpad community) are writers too. They do not want to comment on your story, they want comments on theirs. So, unless you do a review / critique swap, you're not going to get much in the way useful input, if any.
Two. Most active readers never leave comments, and if they leave comments it will be along the lines of 'wow', 'never expected that', 'cold one, isn't she'. And those are the good comments. Some people will do their utmost best to totally deconstruct your story, spam negative comments, and just troll for the sake of trolling. Instead of improving your writing this will only lead to negativity, despair, and many potentially great writers simply give up. The Internet isn't a nice place...
Three. Your story may simply not 'gel' on your chosen platform. For example, Wattpad (besides having gone extremely commercial) has a. millions of stories, so yours may never show up, and b. primarily focuses on teen fiction, think werewolves, alpha beta omega stuff, girls kidnapped by the handsome mafia's boss's son, and the like. If you're writing a historic novel you won't find any traction there.
Four. Free audience doesn't pay. In general, the quality of most of the stories on platforms such as Wattpad is lower than a regular (published) book, which is quite understandable, as most are written by hobbyists who do not spend money on a professional editor (and why would they). And the readers that flock to these sites accept that level of writing, thus do not see the need to start paying for something on Amazon. In other words, most of the readers on these online platforms will not buy books, but even worse, if your story picks up and attracts millions (you wish!) of readers they might not even notice the bad parts in your writing. Duh!
Note that some platforms are primarily focussed on feedback, such as Scribophile. Those might be a better option if you're trying to improve as a writer.
Becoming popular (attracting readers)
That one's easier to explain, and harder to accomplish.
One. You have to write a story in line with the taste of the audience.
Two. Create an attractive cover and blurb, as that is the first thing a potential reader will see.
Three. Game the algorithm. Each online platform has their own algorithm, their own method of bringing certain stories to the attention of the readers. They will assign a 'score' to your work, using a (complex?) calculation of reads, review scores, number of reads last week, total reads over the life span of your story, counting keywords, votes, time spend on each page, and whatnot.
If you don't 'game' the algorithm you're pretty much never going to be discovered.
In other words, attracting lots of readers is a kind of populistic political game, nothing more, nothing less. Smile at the camera. Tell your voters what they want to hear. Get involved in fora, if there are any. Use any and all social media you can find.
In general: sell your soul.
(Repeating myself.) The quality of most of the stories on platforms such as Wattpad is lower than a regular (published) book, which is quite understandable, as most are written by hobbyists who do not spend money on a professional editor (and why would they). And the readers that flock to these sites accept that level of writing, thus do not see the need to start paying for something on Amazon.
In other words, most of the readers on these online platforms will not buy your books. Perhaps you can sell them the sequel, leave out the last ten chapters and reserve those for the commercial, complete version on Amazon, perhaps you can write a side story to attract interest. There are options, but none of those offer immediate financial rewards.
How much money is going to be made anyway?
Let's say you have writte something, and hire an editor for $2000, and an artist for the cover for $200, you'll end up spending 'x' hours plus $2200 to get your book publishable. Now release it on Amazon, and assume you get the highest revenue percentage of 70%. Then you would have to sell at least 789 copies at for example $3.99 to break even.
$2200 / .7 / $3.99 = 789
It's hard to get real numbers, but some authors report selling maybe 60 to 100 copies in their first month, that way they won't ever break even.
Even when you sell 600 copies in the first month, and 1200 over time, you'd only make $2588 netto. Not too bad, but how much would that be per hour? Let's say you spend $2200 on cover and editing and worked for half a year on your story (1008 hours).
1200 * $3.99 = $4788
$4788 - $2200 = $2588
$2588 / 1008 = $ 2.56 / hour.
And that's excluding any marketing or such. With a million of novels out there, how will yours stand out?
Flipping burgers at McDonalds probably pays twice as good...
Fun, fun, fun!
Now, that's they key point, after all the negativity above. If you enjoy writing, and if you like to give those few readers who enjoy your story a good time, then keep on doing it!