I've just joined Instagram. I tried a few months back, wasn't smitten, but trying again. It's like a cross between Pinterest and Twitter, but with none of the usefulness to authors that Twitter has. For example, you can't include links in your Instagram posts, so you can post a book cover but no seller information, which seems a little pointless.
Another thing I seriously dislike about Instagram is that it's set up so that you can only make posts from your phone, whereas, as a writer, I spend my life at my laptop. There is a hack around, but my goodness, it's convoluted.
So, all things considered, it's a long way down the list of useful for writers trying to share their work.
Instagram does have a larger following than Twitter, but an audience of people more image than word oriented. So, people willing to engage with Twitter are probably more an author's audience. It all boils down to my post on Sociela Media Marketing, is it Worth the Effort?
I'd be interested to hear from other writers what you feel is most beneficial? I find the #WritingCommunity tag on Twitter very engaging. They recently ran #IndiApril, and I know I sold a few books through that. An inspired campaign. Anyway, whether Instagram is useful to authors or not, for the time being - pretty pictures. If you're on Instagram, drop me a line and connect: authormgw.