I have used both Tapas and Wattpad to grow my readership, and for me, neither platform is that great. Of course, before the wolves jump on my case, hear me out because I'm only speaking from my experience.
To make things easier, I've separated my thoughts below.
Wattpad - From fresh eyes, Wattpad seems like the land of milk and honey for writers, but beneath the layers, lives many issues. For one, Wattpad has now become oversaturated with stories. Because of that, it's harder to gain a readership unless you write in the popular genres, like teen fiction, romance, vampires or werewolves. The website always, always has bugs, and the company makes questionable decisions. For example, they originally told non-Wattpad future members they wouldn't have ads in their stories and now, all of sudden, everyone has playable ads. From what I'm aware of, no one was told this in advance and it just happened one day. Of course, not everything is all bad. If you make friends with other writers, especially popular ones that can promote your stories, you may get a semi-decent following there (500 to 1k followers). As for myself, I left Wattpad about two years ago, and it was one of the best decision I ever made.
Tapas - As a writer here, it's hard to grow a readership, BUT I don't promote my work on the threads anymore nor am I really active on the forums here. With that being said, my experience will be different from others like @375. I have noticed that when I first started on Tapas (before premium novels got their own section) I was getting at least 1-4 subs a week, but that has decreased since the layout changes. Now, that could be because of my story, who knows, but I feel it's harder for readers to find free content now. Another thing is when I did get New & Noteworthy, I gained 70 subs for one story and barely any of those readers read the story, but yet, they are still subbed to it
Overall, I say you get out what you put into it, but it is work (like with most things). I've been on Tapas for almost a year and have 257 subs. It's not great, but it's not too bad. The best thing I can say is don't expect anything from using these websites, and make your plans for after you finish your stories, whether that be self-publishing or traditional publishing.