I feel like part of what caused it to explode is that some celebrities joined up early and it provided a tangible way for fans to interact directly with them. This was totally novel and completely unprecedented. Prior to Twitter you could watch Entertainment Tonight and read People magazine. Twitter let you directly contact celebrities.
I really couldn’t care less about celebrities, which I have always thought is part of why Twitter has always felt kind of pointless to me.
But I fully recognize that for some people that’s a huge draw and I do think it’s a big part of what makes Twitter popular.