I've only done promoting on the Tapas forum so far, and I want to know other ways to promote my comic.
Twitter, Instagram, reddit, cross promotions, Tumblr, Facebook, toptenwebcomics, comic fury forums, discord, etc.
If you are doing a gag comic, you could break down the panels and repost them on Instagram, Twitter, and Tumblr.
I'm using Discord, Twitter, Reddit, DevianArt for promotion.
And ofc forums... You can post comic even here.