Bad posts on social media are probably part of what makes them addicting
Video games & behavioral addiction
The first truly addictive game I ever played was the original Diablo, way back in 1996. I had always liked games, but when you played Diablo you would actually lose time. I mean, really lose it. You'd look up and it would be midnight. The whole night would be gone. I didn't understand, but I knew the game was different. My friend and I commented on it. I liked Doom, and things, but I would get bored after a couple of levels. Sitting and playing Doom (at the time) was actually pretty tough for me. I didn't want to play past 15 minutes. I wanted to go do something else. I even liked RPGs at the time, and I could sit and play those for hours. But, I never lost time with them. I felt the time pass. I really knew and felt that I was sitting there for 2 hours playing. Diablo was different, and we could easily feel the difference.
Behaviorism
I graduated college in 2006, and despite never actually using it in my career, I did actually get a bachelor's in psychology. In one of my courses, we studied behaviorism and I learned about schedules of reinforcement. This was explained to us with rats in cages that press a lever to receive sugar water. ie, a reward. Interestingly, the way you actually reward the rat has a big effect on its behavior. Think about it: you could give the rat sugar water every time it presses the lever. You could give it every 10 times it presses the level. You could reward the rat after 30 seconds of pressing. etc. Interestingly, the most addicting behavioral pattern is the variable-ratio reinforcement schedule. The rat presses the lever once and gets a reward. Then it presses it 10 times and gets nothing. But then on the 11th time, it gets a reward. Then 5, then 30, then 100, 2, etc. How many times the rat needs to press the lever is variable. The theory behind this approach (at least, going from memory) is that the rat just not know when the next reward could come. It could always be the next press. It almost never is, but it could be. So there's always a reason to try one more press of the lever; the big pay-off could always be next. The times the rat presses the lever and gets nothing is a feature of the addictiveness of this approach. You can reward a rat every single time it presses the lever, and it absolutely will not make the behavior nearly as addicting as if you use the variable-ratio schedule.
Obviously, this maps to slot machines. Every coin inserted and every pull of the arm could be the next pay-off. Of course, usually it's not. But sometimes it is. And just like with the rat, that is part of what makes the slot machine so addictive. The uncertainty and disappointment are surprisingly a feature, not a bug.
Back to video games
Once I took this college course, I realized why Diablo had been so addicting. It has random loot drops. You kill and enemy, and there is a chance for a weapon or item to drop. Sometimes it will drop nothing. Sometimes it will drop something. Very rarely, it will drop something very valuable. So the more you play, the more chances for a good drop you have. It's addictive in precisely the same way that a slot machine is addictive. My college course finally explained why I could lose time in Diablo rather than merely enjoy it.
Once you see this pattern it's hard to miss it. Diablo feels like it accidentally employed this feature. Diablo II, released in 2001 was clearly aware of this and more heavily built loot drops into the gameplay. People play Super Mario World to have fun. But Diablo II? People just play it to grind for loot. A secondhand market for this loot actually came up, because it was so "valuable." Its value was merely in its scarcity. There was a small amount of PvP in Diablo II, but for the most part even when people finally grinded or purchased their loot, they just went back to grinding. The whole game existed to addict you and waste your time. And the people who played Diablo II the most just grinded.
Not everyone seems to be able to see this pattern
Around 2009, while I was still fairly young, I had a roommate and a local friend. We'd play video games together; mostly the Halo 3 demo on his XBox 360. This was around 2009, and the internet and games were just beginning to become truly behaviorally addictive. The "Diablo" and "Diablo II" patterns were becoming more obvious. Mobil games, originally a novelty, wasted no time in fast-tracking to the behavioral addiction strategy. More and more games started making use of this as well. Even as a young man, I could see that these games were dangerous and would waste your life, and I could see how prevalent behavioral addiction gaming was becoming.
So we were three friends. Myself, with a background in psychology. My friend Will, a former alcoholic and still an addict. And my friend Rob, who shared none of these traits. Will and I discussed this behavior addiction trending gaming. We had a dumb shorthand joke for it: "crack points." I wasn't the most clever joke, but the point was that the behavioral addiction games were a bit like crack; addictive in their own right, and dangerous. Our friend Rob really didn't get it. If any game had any reward system whatsoever, he'd try to share our joke with us. "Ahhh, crack points!" he'd say if he got something like a high score.
Will and I would roll our eyes at each other a bit. It was obvious to us that Rob did not understand. It was obvious to us that there was a world of difference between a game intentionally employing behavioral addiction techniques, and a game that just had some sort of reward system in the abstract sense. "Beating the game" or "getting a high score" or even "losing yourself in a beautiful, virtual world" was not the same as playing the slot machine. But Rob could never seem to understand.
I bring this up not to be rude to Rob. I bring this up because many people also don't seem to understand. When you try to explain that social media is addicting, they think it's just like when people watched too much TV, or that it's just like any hobby, except it's just not good for you. I'm not sure what would make them understand, but anything that employees behaviorally addicting techniques is bad for you, and does not have your best interests in mind.
The shitty posts on social media are intentional
(even if the companies figured it out by accident.)
So, what does social media have? A bunch of trash, and infinite scrolling. The next post is available to you with effectively zero effort. Like the rat in the cage, social media would actually be less addictive if every post was something you cared about. No, you scroll, and see crap. You scroll more, and see more crap. After enough scrolling, you see a really enticing post. Enticing can mean one that stokes outrage, one with sexual content, etc. Anything that provokes a reaction. Just like the rat in the cage, you don't know when that next enticing post will come. So you keep scrolling. Maybe it comes after 3 posts. And next after 20 posts. And after that, 5 posts, and then after 50. etc.
I do wonder that social media companies originally did this on purpose. This feels like something that could have been discovered purely by accident. It feels like something which could have been deployed without understanding. Some PM had an opinion about web UI, and felt clicking to change pages was clunky. I'm just speculating, but I really bet they just wandered into this. At least first. By now, I'm sure they know. They know and they don't care, because it makes them money. Once you combine infinite scrolling with variably-interesting posts, you have a skinner box.
This is definitely not the only behavioral addiction employed by these companies. Just the one I have some insight into.