Currently, it’s 1.5 million people: http://www.facebakers.com/countr…. Facebook translated its UI into Japanese in 2008.
For Facebook, Japan is one of the few remaining white spaces on the world map, next to South Korea, Russia, and China (where it’s blocked).
By way of comparison:
Japan’s homegrown social network, Mixi (http://www.mixi.jp), currently has 21 million users. Mixi was launched in 2004.