Any given Instagram reader may not have read a real book in their life - or maybe they are avid readers who read constantly. All the same, there will always be readers and non-readers.
Literary snobs today imagine a past world where everybody was constantly discussing the finer points of the classic masterpieces every day of their lives. In reality only about 12% of the global population in 1820 was literate. And that's just baseline, not to speak of people who actually spent time reading difficult, thoughtful, full-length books.
Thoreau has a great passage in Walden (published in the 1850s) where he complains about people knowing how to read, yet spending their time reading garbage romance novels instead of classics like the Iliad. I think the complaint of people "reading books" on Instagram comes from the exact same place. It's a timeless problem, which is not really a problem.
Even when global literacy reaches 100%, there will still be a very small percentage of people who are hardcore readers. There is a certain kind of person who will sit down with a big heavy paper book and read it cover to cover, and they are not a common species among humans. It's something you are born with, or something that you gain in very early childhood anyway, and nothing Instagram does will ever change that.
So there's really no use in complaining about it one way or the other. Instagrammers will instagram. Some of them will read real books too, and others will not.