Hey figured I would come in and share some advice:
My favorite self-promo threads are the "share your latest page" threads, theres usually a new one each month. Here's the current one:
Other examples would be "Share your comic under X subscribers" or "show me your horror comics" - they're all around the forum if you dig around a bit you should be able to find them.
All that said - self promo threads (with the exception of the share your latest page thread) are not that useful IMHO, a lot of the time they come off as spammy. You're really just better off participating in community discussions and dropping a link to your comic when it's relevant. (Also, link your comic in your forum bio, if you check out my form profile you'll see what I mean.)
When it comes to self promoting elsewhere you should follow the 80/20 rule. 80% of your posts should be "content" and the other 20% can be "advertisements".
When I'm on the ball with my instagram I usually post a handful of sketches or drawings over the week, and then on monday I have a post showcasing my latest update.
On Twitter there are artist support threads and FF posts, as well as the #webcomicchat
Best of luck!