Honestly, I haven't gotten around to using social media to promote my novel. I rely mainly on promoting through this forum and working with Tapas' algorithm when uploading. I hear from other writers that social media hasn't been the best in attracting an audience, however, it still doesn't hurt especially if your readers want to follow you. 
If I did start an Instagram though, aside from promoting my novel, I would also create content that could help aspiring writers and/or readers.
For example, you could share posts or graphics about writing tips, writing for an international audience or blog about your journey in growing an audience as an online writer.
Have you got Twitter? I know a lot of Tapas writers use it, so you could follow writers that you like and see what they do to promote their novels. Probably more helpful than what I have to say 
Hope this helped in any way. 
