Om reports that Softbank is bringing in MySpace to Japan.
This will be interesting because MySpace is much less rigid in controls for the user than Mixi, the current leader.
I think Mixi/eMercury COULD be significantly challenged by MySpace, especially if Softbank will leverage their relationship with Yahoo! Japan (I cannot imagine a scenario where they would not.)

MySpace is also launching in Japan in partnership with Softbank. The 50:50 joint venture is expected to launch sometime in September 2006.

It will be interesting to see how quickly MySpace Japan grows and whether the platform is to the taste of Japanese youth (who are less Internet savvy than their American counterparts- i.e. in terms of customizing their pages, etc.)
EDIT: A later thought that I had surrounds Mixi and the private nature of that SNS. If MySpace launches without the privacy features that Japanese Internet users want (i.e. 90% of Japanese blogs are anonymous, etc.), if MySpace Japan is merely a UTF-8 localized version without customization for the Japanese market, I don’t see them succeeding.
Japan has half the population of the US, and Mixi is less than half as large as MySpace, but it’s still a formidable competitor, moreso if they get their IPO done this year.
GigaOM : � FIM Revenues – $500 million in 2007