It is not the
content on social media that is doing the harm in Haidt's model -- beyond the fact that the content is designed to be more stimulating and more interesting to that particular user than "real life".
Haidt argues that social media platforms are designed to be addictive,
leveraging variable reward schedules and constant novel stimuli (likes,
notifications, new content) to continually trigger dopamine release. This
overstimulation of the brain's reward system, which is heavily mediated by
dopamine, can lead to several negative consequences.
This constant seeking of instant, shallow
rewards trains the brain to crave immediate gratification, making it more
difficult for young people to engage with activities that require sustained
attention, effort, and delayed gratification, such as reading a book, focusing
in school, or building deep real-world relationships. The constant
overstimulation can lead to a dysregulation of these pathways, making it harder
for individuals to feel motivated by or find pleasure in less stimulating,
everyday activities, thus contributing to feelings of boredom, apathy, and a
diminished capacity for sustained attention and effort.
If the same exact content were revealed to the user in response to the user's making an effort, undergoing some danger or exercising patience or forbearance, the adverse effect described above would not occur: what causes the adverse effect is stimulation and gratification that requires almost no effort, no risk, no waiting and that is constantly available (even for example when the user is standing in line or riding on a crowded train).
The same thing is true BTW of the effect of porn on the young mind: if a boy can persuade a girl to show him her body, that experience will not tend to damage the boy even if he sees exactly the same thing he would see if he watched a porn video because it almost always requires significant effort to persuade a teenage girl to take off her clothes: among other things, the boy would need exhibit some degree of consistency and predictability in his behavior (exercising his frontal lobes) and to show that he cares about her at least to some minimum standard, which will probably entail learning and remembering many things about her and her recent experiences.
Almost everything in life that is pleasurable or exciting requires significant effort, danger or waiting, not just persuading a teenage girl -- or at least that was the case before the widespread availability of paperback novels, newspapers, comic strips, easily affordable distilled alcohol and drugs, radio, movies, pornography, television, the internet, social media and smartphones.