I too have a fairly high volume of Japanese Traffic. I send mine to Adult Friend Finder. They have Japanese full page ads and banners to use for their program. Plus, thier main site will detect the language of your surfers and automatically send them to the Japanese language site.
I wouldn't recommend sending it to Asia Friend Finder, their mainstream product. It doesn't convert nearly as well as Adult Friend Finder...their Adult program.
Also, opt for the Pay Per Sign-up option. Over time, you will make much more with this then with the per click program. I use a number of tracking URL's and I am still making money from poeple that signed up for free weeks, months and even up to 2 years ago.
If you give it a try, please do me a favour and signup using my affiliate ID.
Adult Friend Finder Affiliate Program