Most Japanese dislike to join fb simply just because it is still English sns, as you don't feel good if you are in mixi, fully in Japanese.
Then I'm curious, why we Japanese love Twitter...
Probably because Facebook decided to "invade" and set up office, whereas Twitter worked with Tokyo-based Digital Garage to localise Twitter for the Japanese market (and attempt to provide extended services geared for the Japanese market). Twitter also doesn't require your real name - all they care about is that impersonators specify that they are parody and not genuine.