What’s noteworthy here is that the site didn’t update the number for Japan for over 2 weeks (it was stuck at 4.8 million for 2-3 weeks), which is highly unusual.
Another (more) interesting point is that Socialbakers’ number is simply off: according to Facebook Japan country growth manager Taro Kodama, the site hit 5 million active users in Japan on September 15.
Socialbakers is just counting registered users so it should display a higher number for the Japanese market. As I previously said, the service pulls numbers from Facebook’s ad creation tool, which also currently shows 5.03 million users for Japan:
The difference isn’t really surprising, as Facebook itself says the number from the tool is an “estimate” and “not exact”.