Facebook isn't a serious player in the Japanese social networking space. They are dwarfed by the rulers of the Japanese social web - Gree and Mixi, but its clear they are serious about gaining marketshare.
Yesterday when I logged into Facebook from my hotel in Tokyo I was presented with an option to link my Mixi account. Today I was presented with another Japan-specific feature on the top of my page:
Notice: "We've been working hard to make Facebook Mobile usable in Japan. There is still a long way to go but most of the basic functionality is there.
Please check it out and let us know what you think."
It seems like everyday I'm presented with an invitation to use a new Japan-specific feature.
Facebook currently has a better desktop web user experience than Mixi. Their UI has an AJAXy, modern feel while Mixi's UI feels more like a 2005 web app. That being said, Gree and Mixi have a lot more experience with mobile social which is crucial in a country where two thirds of all social media interaction occurs on phones.