While the average number of shared posts per user is down (theoretically because folks are following more brands than before) check out these other Instagram stats:
"The amount of time that people spend on Instagram also has held steady since 2014 at an average of 21 minutes per day, the company says."
"Overall, usage of Instagram is looking healthy. The number of monthly active users hit 500 million last week, up from 100 million in 2013. Sixty percent of monthly users now open the app every day, up from 50% at the beginning of 2013."
"Instagram has noticed that people share more when they see more of their friends’ posts..." Link