I wonder if the old forum model would work. Instead of these mega-forum-platfroms, there are just small communities with a niche focus at their own URL.

I suppose bots could find forums that use the most popular software and still make accounts and spam, but it would be much more obvious and less fruitful for someone to spam deck builders in Vancouver (something I saw often on Digg) on a forum that is focused on aquariums owners in the midwest.