And of course this is by deliberate design. From the link: "Most social media sites create irregularly timed rewards . . . a technique long employed by the makers of slot machines, based on the work of the American psychologist BF Skinner, who found that the strongest way to reinforce a learned behaviour in rats is to reward it on a random schedule." I think it was Facebook or Instagram that actually withheld "likes" to notify the user later on in a deliberate rush.