I started out a long time writing chat roleplay; it was this continuous amalgamation of ideas that never seemed to stop for years and years — villains rose and fell, new world tones came to light as technology and society changed with the coming and going of a multitude of writers, and for a time it was the only thing I ever wanted to do. We had a forum where stuff happened, too, but it wasn't nearly as active.
And then we started running dry of writers. It took a long time for things to finally die, and even with people walking out for lack of roleplay there is still the occasional scene or nostalgic monologue that people leave in the logs from time to time.
So I started looking for a change of pace — something with a bit more genre freedom, because my first roleplay ever guided me several years with the same overall theme — and found RPG, then RPN, and finally Iwaku, which I've settled in. There was a spat of DarkMyst and an obscure forum (whose name I can't remember but I recall playing a surprisingly entertaining Doctor Who/Star Trek anthrofied RP on) somewhere in between, but that never worked out.
Since I've started to roleplay on forums, I've discovered that I love that I can pretty up my posts a lot. And also that if I want to I can write for ever and ever and ever and nobody will complain, but I've also noticed that I'm missing the old pace and variety of chat roleplays — that we could just throw out a new plot every time we thought one up and if it was awful then our characters would just conveniently not worry about it and if we really enjoyed something we'd beat it against the wall until we'd carved a bloody equine pictogram all over the wood ...
Okay, that last bit was getting sort of winded.
Iwaku having roleplaying chatboxes was the biggest reason I stuck around at first, I think; and when I started really getting comfortable writing forum posts I think I was sold for good. And still am. :3