Word of mouth might just take care of itself when the product or service is remarkable like YouTube. No adaptation to specific local market needs. The following chart is the comparison of community size for YouTube vs. Google Video inside Mixi. 
UPdate: Thanks Ben for pointing me at http://www.japanprobe.com/?p=238
Update: For the record, when Google acquired YouTube, October 11 (Japan time), 2006, its community inside mixi has grown to the 2nd larget community with the membership of 124,679 (at the time of this writing), 736 times bigger than it was 166 days ago (April 29).