you'll probably get more visibility for original art on deviantart, and there's a pretty strong culture of you watch me/I watch you. make friends, engage with others and their works, and you will probably get decent engagement back. there's tons of groups to join and plenty of ways to participate, so despite how weird DA can be, I still think it's a p good site.
tumblr favors fanart generally. I'd say most people on tumblr are there for fandoms, so if you do fanart you'll get eyes on it pretty easily if you tag right. but sraly, tag it right. tumblr people are....a sensitive bunch. and a tip - people respond to things on tumblr with tags more often than they'll actually reblog with a comment, so don't forget to check tags!
I don't know as much about instagram, but for me it seems more like a, if you want to see my art and nothing else, here it is.