Three-quarters of Japanese social network users access the sites only from their mobile phones, according to a survey conducted last year with almost 4,000 social network users in Japan by Mobile Marketing Data Labo. In Japan, mixi and Mobage-town and Gree are among the most successful social networks. Mixi has a reported 17 million users in Japan, compared with 1.4 million Facebook users and about half a million Japanese people on Twitter.   More details on mobile social networking in Japan, and a link to the complete study, at SocialMediaToday.