Metafilter is a pretty interesting case -- deliberately limiting growth seems to have worked out pretty well for the site over most of its existence, and growth

Metafilter is a pretty interesting case -- deliberately limiting growth seems to have worked out pretty well for the site over most of its existence, and growth goals have always been about other qualities than sheer numbers. It doesn't need to compete with anybody else on scale. I'd say HN is one of the few forums competitive on quality of discourse vs metafilter, but its business model is essentially loss-leader for VC/incubator activities, so it's a bit of an odd duck itself.