Well, this might be considered one of the major English-language forums on Japanese topics, it's presence is practically unknown within Japan itself. We have only a handful of native Japanese members residing in Japan, with a few more foreign residents, and a vast majority of our membership is made up foreigners from the rest of the planet with varying levels of interest in and knowledge of Japan.
I believe 2ch is one of, if not the biggest Japanese forums. I'm not sure about other forums, but social networking sites like mixi are also extremely popular.