Just a few things to add to this interesting thread.
I think some guys have it super easy in Japan, they just have the right look and a knack for dealing with Japanese girls, there are litterally not enough hours in the day for these guys to handle all the girls that want their attention. I know 2 such guys, and us other 'avergae' guys have had to hammer it into them that it is only that easy for them not for every single gaijin. Some of the more optimistic posts in this thread sound like they are written by guys in the super easy category.
That said I'd agree that for pretty much any gaijin guy Japan is gonna be easier than home.
Well I say Japan, but really urban Japan. This whole nampa thing is all well and good in Tokyo where there is anonymity, numbers and a more cosmopolitan outlook. Out in the boonies (read any city with less than 1 million population) I don't think it's any easier for a guy to hook up be it casually or long term than their respective homelands (unless they are from some backward-ass country where men and women don't mix/date).
One of the reasons that nampa can work is because the women that respond to it simply wanna have more sex. Like other places, women who want more sex are unfairly looked down upon in Japan, so within their peer/social groups that are carefull to not appear to be sluty. If they are walking alone around a busy (annoymous) part of Tokyo (or similar big city) and a random dude approaches them and they like him, well, it's a chance for them to have some extra sex with no consequences and a love hotel is never too far away...
Of course most women are still victims of social conditioning that prevents them from doing this kind of thing
Regards adult friend finder which was mentioned a few times, same rules apply as using that site from any location - it's very competitive, unless you can put together a good profile and send decent mails that go beyond "hey baby wanna fuck?" or "kawaii, asobitai?" you will get no joy, if you have a solid profile then you might do quite well on there, but I still reckon you get maybe 1 meeting per 100 messages you send.