Heya! My comic currently has 30k subscribers on tapas and webtoon each, and though most of them I would assume came from the site itself (features, trending, etc) I also promote heavily on social media! I'll share what little I know, I hope it helps!
In short I think the "best" social media to promote your comic is... as many forms of social media you have time to consistently promote your comic. Consistency is key in building readership/followings!
Twitter - You can't use a ton of hashtags due to character limit but there are hashtag events such as #webcomicchat, #webcomicupdate (a webcomic update account follows/RTs this hashtag). Follow other webcomic creators and interact with them! Twitter feels more like a creator/community space, where you're going to find other webcomic creators who happen to read webcomics themselves, like this forum!
Instagram - More hashtags! People follow hashtags! #webtoon, #webtooncanvas, #webcomic, etc etc, I tend to hashtag the heck out of my updates here, because people do follow hashtags!!
Discord - There are comic creator discord groups out there! And you're able to join, promote your stuff, get in with a community, etc! (One is Comic Tea Party, link to their discord in their bio: https://twitter.com/ComicTeaParty)
The Webcomic Library - https://tagpacker.com/user/webcomic.library I've gotten readers from TWL and the person/people who put this monumental list of comics together will ALSO promote your updates on Twitter (follow them @WebcomicLibrary)
Ink, Sweat, and Tears - Weekly webcomic magazine you can submit your comic to. I don't know much about them but they seem to be growing! http://www.inksweatandtears.com/
SpiderForest - Their applications JUST closed at the end of June unfortunately, but they're a comic collective! You need to apply and have your comic approved by a jury, but they also provide you with a website of your own, and advertising on their collective website, all while retaining the right to post your comic anywhere else you want--it isn't a publisher, it's just a community with some real big perks if you can get in. It's really neat! https://spiderforest.com/
Tumblr - Used to be alright to promote original content, now it feels like I'm shouting into an abyss, don't bother
TikTok - I am REALLY hesitant to rec tiktok due to all the nasty things I hear about how it just scrapes your phone for data constantly, but it does have a small webtoon-reading community. A large portion of tiktok's userbase is under 25 and I find webtoon also skews a bit young so this is probably connected. Fellow kids, etc
Edit:
A frustrating aspect of gaining readership is that it feels a little exponential. It probably took me the same amount of time to get the first 5,000 subscribers on any platform than it did the last 25,000. The more people read your comic, the more common it's going to end up on trending and popular and the more people are going to see it and subscribe, etc, etc. Features can feel random and luck-based and I feel very lucky to have gotten into collections on tapas and on canvas features on webtoon. Growth isn't linear and it isn't the same for everyone, but I hope these tips help.
This got long. Oops!