This isn't really an alternative to Instagram, but Twitch has been gaining more popularity within the art community lately. There aren't as many artists or art viewers on there compared to other networks, but the audience is much more invested.
I tried it out one day not expecting to have anyone tune into my stream, but to my surprise I actually had 1-2 people start watching and asking me questions about my art and my process. It was pretty cool.
It definitely takes a certain personality type to live stream, but if you're the kind of person who likes to talk to themselves while drawing, it might be something to consider.