Data doesn't lie is, in itself, a lie. Data has to be collected, so it's biased by what you choose to measure. Data has to be interpreted, so it's biased by how you interpret it. Data is only a snapshot in time, so it doesn't predict the future. There are many, many ways to make engagement metrics look much better in the short term while absolutely destroying long term engagement. Whether that's what Instagram is doing is still up for debate, but I think there's a very real chance it is.