Not many people even in Japan know that Yahoo Japan is running a social network: dubbed Yahoo Days, the social network is embedded in the Yahoo Japan ecosystem of sub-sites and was established in 2006 (its used to be called Yahoo! 360° in the first few months).
But now Yahoo Japan announced their social network will be shut down gradually, starting from October 3. From November 30, users (if there ever were any) won’t be able to access Yahoo Days anymore (before that, users can export their blog entries to Movable Type).
I wouldn’t be surprised to see Yahoo Japan buying Mixi and making it part of their empire at some point in the future – it would be good match.