The one that is easier to develop for will likely get more features which leads to more users.

Not necessarily. It might get more developers at first when people think it's going to be the Next Big Thing (TM), but if nobody uses it, the devs might not feel their effort is worth it and might move on.

Why wouldn't people use it, despite it having "more features"? Because social media is mostly driven by network effects. People go where other people go. All the people there create content which gives people a reason to go there. In the distant past, Facebook only grew because it was so easy to move from MySpace. And, it was easy to move from Friendster to MySpace, and so-on back to the origins of social media. Since then, the walls of the walled gardens have become much higher. Every social media company actively makes it difficult to move to other platforms because they want to keep any users they have. You might hate Facebook, but you like Aunt Jane, and she's only on Facebook, so you stay on Facebook.