Yahoo! Japan (and its fellow Softbank) is incredibly popular in Japan, and plays a top role re Internet: ISP, search, mail, cell phones (SB)...
Question is "why?" Why Yahoo! (mail, search,
...) are still that popular? Why the Japanese didn't follow the Google trend (as much as the Western countries) during the mid 2000, where Yahoo! had (and
still has) those very busy pages and (for a long time) the search was of a much lower quality compared to Google, having a lot of results being sponsored by 3rd parties displayed on top without further indications about that sponsoring.
There is a technical answer: most PCs come with Yahoo! stuff, the search is set to Y! and nobody changes that. The thing is, compared to the West, the Japanese do not have that "pursuit of genuineness" reaction - they trust what is popular and Y! is very popular...
Furthermore, there is no strong consumer association in Japan, and abuses (in any field) may remain undisclosed for a long time (yes, there is a connection between Y! and abuses).