WSJ provides an overview:
Yahoo Japan and Rakuten are still the nation’s dominant Internet sites, but rivals are gaining with services the two don’t offer.
Mixi, for example, runs Japan’s largest online community site, whose users have quintupled in the past year. Now, nearly six million people use the site to network with those who share similar interests. The company in May became the third-most-popular Internet site after Yahoo Japan and Rakuten, according to NetRatings Japan, a market-research firm that rates Internet usage. While Mixi still lagged behind its bigger rivals overall, the average Mixi user was found to spend more than an hour longer per month on the site than the average Yahoo user.
Other companies are also seeing big gains in customers. Broadband content provider Usen’s GyaO site offers a popular service that lets users download movies for free while CyberAgent, an Internet advertising and media company, has seen strong growth in its blog portal site Ameba, which gives users access to more than one million blogs. In e-commerce, Rakuten faces competition from Amazon.com’s Japanese site as well as others such as hotel booking site Ikyu.