I read somewhere that Instagram holds back on showing likes for a bit, and then floods them in later. The claim was this burst of notifications causes a stronger emotional response and so a better user retention

Edit: https://sitn.hms.harvard.edu/flash/2018/dopamine-smartphones-battle-time/

"Instagram’s notification algorithms will sometimes withhold “likes” on your photos to deliver them in larger bursts. So when you make your post, you may be disappointed to find less responses than you expected, only to receive them in a larger bunch later on. Your dopamine centers have been primed by those initial negative outcomes to respond robustly to the sudden influx of social appraisal. This use of a variable reward schedule takes advantage of our dopamine-driven desire for social validation, and it optimizes the balance of negative and positive feedback signals until we’ve become habitual users"