This is based entirely on my own experience, I can't say if others are the same.
Facebook: total waste of time, IMO. Most of the groups are just the same people, and overwhelmingly just creators, not readers.
Instagram: I post at least once a week if I can, and this brings me... eh, about three or four readers each time.
Tiktok: If you have the time to make videos for this, it's a great resource. I've gotten about 300 or so subs just from tiktok with almost zero effort. I have like maybe three whole videos on it. Problem is that I really don't have time to make them.
Youtube: I sometimes post shorts or speed drawings here, but very little actual viewers bother to check out the comic. Again, it requires time to make videos and I just don't have it.
Reddit: This is my bread and butter. I've amassed a couple thousand subs on my Webtoon version of my comic almost entirely from posting in subreddits. It's all about posting in subreddits that have to do with your genre or things featured in your comic. Don't spam though, and actually be somewhat active in the communities or they'll boot you for spamming. I post once per week. Don't just post your story. Post a scene or something actually relevant to the subreddit. Then put your blurb and a link to your comic in the comments.