When texting took off, it was the easiest (only) way to send instant text based messages between friends wherever you were, even if the phone system is now heavily used by spammers and there are better options.
When Facebook took off, every Myspace page was so full of garbage that they barely loaded on most people's computers, and Facebook was slick and shiny and easy. The real name policy made it super easy to connect with people you met IRL. Even if it's now confusingly slow and FB Messenger can't display your recent chats in the correct order for some reason, it was the easiest most obvious option at the time.
I don't really understand why people use Twitter (at its best it just seems like a worse version of RSS), but the site presumably loaded quickly at some point and was easy to use, even if it's presumably worse now.
And so on. They persist through momentum.
Some things continue to persist, some things get beat out and die. But if you start off more confusing than your alternatives, at least compared to when they started, you won't get picked up in the first place.
> I don't really understand why people use Twitter
The honest answer is that it isn't the content(RSS feeds), but the combat sport nature of the platform. It's the only place where you can tell a billionaire any kind of awful thing you can think of. It's also the only social media that drives important people insane. The wealthier they are, the more insane they'll be driven.
Facebook will drive your meemaw insane with AI generated ads of legless veterans being given a cake.
Twitter will drive the richest man on earth insane. It will drive every journalist at any paper of merit insane by interacting with the insane billionaires. Nearly every journalist who uses twitter enough will develop delusions of grandeur that their brand of psychopathy is the solution to the nation's woes. Since their bosses have also been driven insane via twitter, it's the kind of writing that gets published. This writing will take the insane delusions of the insane billionaires at face value. It will go along with conspiracy and never beg any question that actually needs answering.
Twitter drives them all insane the same way that we used to marvel at the way celebrities got driven insane by fame. The feedback is going straight to their heads and they are losing touch with reality isolated in their little bubbles the same way celebrities get isolated from reality through their wealth.
Nobody really wants to use instagram either—there's basically nothing positive to say about the app or service itself—it just has critical mass.