The "forum system that is most conducive" to our goals might well be a combination of one very open central site (LessWrong itself) supplemented by a variety of more private sites that discuss rationality in different ways, catering to a variety of niches. Not just Eliezer's Facebook page, but including things like MoreRight, Yvain's blog, Overcoming Bias, Give Well etc.

This makes me a little suspicious as a solution, only because there doesn't seem to be anything particularly special about it besides being precisely the system that is already in place.