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.
Is that unique to the US? It feels like something broadly true if I believe what I see in the news and online forums, but in my personal life it does not feel true at all.
On the other hand, I've talked to a lot of other immigrants who lived in different states before this, and the general consensus (in that admittedly small sample) is that the US is definitely different from South America or Europe in that sense.
For context, I lived in Chile before I moved to the US, and Chile is the country in South America that tries the hardest to be like the US. Even in Chile, it's easier to have a richer social life than here, despite longer working hours and longer commutes, which both result in having much less free time. My own theory is because you get to socialize more at work and, if you have a kid that goes to kindergarten or school, with other parents. Here? "Not so much" would be an understatement.
Twitter's community is the entire world. (as far as I know)