I have always had good luck with shared write pads in one form or another. People reply and then one person can copy/paste to the forum. Helps with flow of interaction in my experience.

One long term game I am still in is only three people and everything is done in google so we can react as needed without a weird break of flow for dialogue or short posts. 

Also it helps if you have a group who demands 300+ words and can't because you are mid conversation with another character. 

Just my experience mind :)