Yes, the number of Japanese JWs has been in decline for well over a decade. It's amazing to consider the stagnation since the mid 1990s, after the rapid earlier growth. The heyday of phenomenal growth of JWs in that country is long past.
As another poster said, the proportion of female Japanese JWs is significantly higher than male Japanese JWs. Indeed, in Japan, religion is very much a female-dominated pursuit even more so than in Western countries.
I think attributing the decrease to the internet lacks evidence. Are there any Japanese-language JW-based websites, especially those that question the Watchtower Society? My guess is that the Japanese have got over their initial fascination with the regimented JW lifestyle and have simply gone back to their own regimented lifestyles!
It would be helpful to hear from any posters who are themselves Japanese or who know from firsthand contacts with Japanese JWs.