It’s hard for me to vote on this, as I was someone who didn’t get Instagram until I got 100k reads anyway 
I think for sure it helps authors who are publishing another story, because they already have gained a following based on their existing story/ies. However, I think if your publishing your first story, whether you have Instagram or not won’t really change how hard it is to first get noticed. It’s hard to get reads when people don’t know you exist, you know?
But I actually did a similar thing to what @EliseC said she did. I published, but didn’t announce it anywhere, I think I got about 30 reads, and then when I announced it a few days later on Instagram (on a Friday too), it got about 1k reads within the next day. So based on that, I guess I can’t deny that it helps to have Instagram