My own anthropological pet theory.
Dunbar's number is the concept that a person can only have so many meaningful relationships. Another way of thinking about it is that we, as a species, feel most comfortable in tribes with a certain number of people.
Which makes sense. I hated, hated the little rednecky town that I grew up in. But when I moved to a larger city, the first few weeks were spent overcoming the loneliness of not knowing everybody around me.
What massive online social media does is essentially short circuit the behavior that we developed when we began urbanizing.
Now a person can be a member of twenty tribes without ever needing to leave their homes. If we are, in fact, only capable of a finite number of close friendships then every close relationship that you have online is energy that won't be spent on a physical one.
True story: I left Facebook in 2016. I had been miserable but didn't even think to relate the two. About a month after I jumped ship I got adopted by a group of fellow nerds nearing midlife. We hang out at least twice per week. Pub trivia, bowling, hiking. Those interactions are so much more meaningful than anything you can get on social media. By our nature, humans crave physical company.
Social media isn't going to "break" us. But, if nothing changes, it will further dramatically alter society.