The thing that ruins it for me is the constant negativity. Everybody is upset about something. Often, rightly so, but everybody is also extreme in their views. There is never nuance or healthy discussion. Only anger. Even in innocuous things. For example, follow the official account for a sport on Instagram. If you ever wade into the comments it's a cesspit of anger and negativity. Even if you never get involved (I don't ever comment) it effects how you feel.
Twitter is the worst offender. I can curate a feed of people I enjoy seeing content from. This should be enough, but these people can retweet things I don't want to see. Fortunately Twitter allows you to disable retweets for people. But...Twitter populates your feed with tweets people you follow have liked or tweets Twitter thinks you might like. The majority of the time these are things I don't want to see that cause negative emotions. It makes Twitter unusable without customisation via Chrome extensions or third party apps.
When I look around at my friends, the people that have the least negative outlook on life and seem to be most outgoing and happy are the people that don't actively use social media or who never even signed up. They're the people who don't know what's happening in the news unless there is a major crisis that might actually effect them. It's always hard to know from sure looking in but these people seem to have the right idea.
When I look at it objectively, I get absolutely nothing beneficial out of the most popular social media companies (or reading the news constantly). There is something about it all though that makes it really hard to step away. It's FOMO more than addiction. I never last more than a few days, but they're always better days.