Just like any other medium, social media in itself is not bad. It helps us connect with people, interact with them, get educated and be entertained.
There are some habits and attitudes towards social media that could be unhealthy. Issues with self-esteem and self-image are well understood. Below are some other issues from a neuroscience perspective.
Most of the social media cellphone apps and social media websites now allow for infinite scrolling. Take Facebook as an example. You login to Facebook, and you land on one home page. After that if you want more content, you just scroll down. You don't even need to click for new content. You just keep scrolling down and you see more and more content. This is called infinite scrolling. Facebook, twitter and most social media now have infinite scroll.
For videos, in the initial days of the internet we had interesting videos. We had one video with one interesting concept or idea or story. Now a days we have compilation videos. You have one video with many short videos compiled together. So single video has lot of interesting things in quick succession.
There is also the popular idea of short videos. Take TickTock for example, where you have videos that last only few seconds before moving on to the next video.
All of these give us continuous pleasure hits. These pleasure hits occur every few times a minute. We get used to getting these hits while looking at 'interesting stuff'. We also get into a mode where we are searching for interesting stuff. You look at a person using cellphone and they are either looking at interesting stuff or discarding stuff that is not interesting or searching for interesting stuff.
This is alright in small quantity. But if it becomes too frequent, it might cause issues with certain networks in the brain. With too much stimulation in this fashion, the Dopamine System (also called the Reward System) gets over used and desensitized.
This causes issues with self motivation or what we commonly call as procrastination. Procrastination means when a person is not able to motivate themselves to start or continue an activity and thus the person keeps postponing it.
Watching videos or photos on social media and watching television are mostly passive activities. These get a person in a mode where they are just consuming media over long periods of time. This causes a certain amount of mental adaptation. Basically the person becomes like a media zombie. This person is able to be inactive for long amounts of time, for many hours a day. Thus the media zombie state promotes passivity and creates mental inertia. Now if the person wants to do an activity, the person first needs to overcome inertia developed due to this passivity.
Infinite scrolling or anything that gives frequent hits is bad. Movies/web series/documentaries are less worse. These provide you longer context, longer stories and does not provide you instant hits. So these are less unhealthier in comparison.
On-line gaming also is better. It challenges the brain, is interactive than just being passive, improves attention and reaction times, improves focus. Games that involve strategy improve mental models and those involving navigation and maps, improve spatial memory. Too much gaming of course is bad. But when in moderation, playing games is actually beneficial for the brain.