In America, you have Facebook and Myspace. In Japan, we have Mixi. Although hardly the world's most innovative social networking site, it is a force to be reckoned with here in Japan, with an installed user base of over ten million people. In fact, it's hard to even find Internet-literate people here who DON'T have accounts. Like any social networking site, it's packed full of communities mainstream and obscure -- and thanks to the fact that registrations are invite only, you tend to get a more modulated, mature version of the discussion that occurs on totally anonymous sites like 2ch.
Update March 24, 2008: in an apparent response to user protests, Mixi has backtracked and revised the offending Clause 18 of their TOC to definitively state that "all rights (including copyright and personal-usage rights) for content belong to the user that created it."