https://news.ycombinator.com/item?id=16101310 (2018)

> "Matt Mayberry, who works at a California startup called Dopamine Labs, says it's common knowledge in the industry that Instagram exploits this craving by strategically withholding "likes" from certain users. If the photo-sharing app decides you need to use the service more often, it'll show only a fraction of the likes you've received on a given post at first, hoping you'll be disappointed with your haul and check back again in a minute or two."

https://news.ycombinator.com/item?id=28763413 also speaks to this.

(Dopamine Labs is now boundless.ai, only one example, the reward hacking mentioned is everywhere in socials, adult content, sports betting, for reward maxxing)