This question needs to be asked more often. It took me quite a while to realize why I'm more engaged on Twitter than I used to be.
In my case, it's to have a conversation. Before I moved to the United States, I could have all sorts of conversations with people at work, no matter how controversial or stupid or weird. But in the US, the culture is different. There are things that you can't discuss with your coworkers, for various reasons.
When the pandemic hit and I stopped going to the office, that made the problem even worse. Sure, I didn't have any real friends here, but at least there was more randomness and diversity in my social life. I love my family, but it's an extremely limited pool of people to talk to.
So I found myself participating more and more on Twitter, on Imgur, and on certain game forums. Which, in turn, had the same impact on me as Facebook used to before I closed my account.
My own, very personal conclusion, is that the society in the United States suffers from a "disease" of alienating people from each other and isolating them, making them turn to social networks to fill the void left by the absence of what used to be normal, every day way to socialize.
Then again, I'm just a sample of one, so my conclusion is almost certain to be wrong.