ive started posting more on instagram, and you get waaay more feedback than tumblr - typically 20 likes and a couple followers per drawing (so long as you tag.)
generally, ive heard people say dont tag too much, and tag divisively - if you draw a character from, random choice, naruto, then have a desc like 'heres #thatninjaboy from #naruto !' and then you could add '#fanart #artistsoninstagram' or add a tag cloud in a comment, eg '#fanart #artistsoninstagram #artistsofinstagram #art #digital art etc etc etc' if you want to use more tags. in the beginning, youll want to use more tags, id say.
also, put a link to your webcomic in your insta description, and direct people there in the caption for anything youve drawn for that.
activity in the community is always good - comment, reply to comments (on your posts, mostly), all the rest. the nice thing abt learning the ropes of instagram is its really on trend, so youll find loads of 'how to be successful on insta' kinda posts on websites like refinery29 and mashable and sweet.
honestly, if you can make shit visually appealing, insta is a lot easier than other platforms. on tapas, a following is a long and hard process, on tumblr you need to take a lot of time making fanart before people give a shit abt your original stuff, but on insta.... art is art and art gets likes.