Facebook and MySpace may be all the rage in the US and Europe, but in Japan, homegrown Mixi is the country’s most popular social networking site. But the site—which started eight years ago as a job portal targeted at twenty-somethings and made a $10 million profit out of $46 million in revenue in its last fiscal year—may see itself superseded if it doesn’t move swiftly enough to convert its online success to the mobile web.

According to the Forbes feature looking at Mixi, it’s got a lot at stake. The site, which has 12 million subscribers, comes in second in Japan’s online ad market, with 15 percent of ad impressions, behind only Yahoo, (NSDQ: YHOO) which dominates the market. It makes money from job listings still, with 19,000 companies advertising in the quarter that ended in September. It also has a premium membership service where users pay 300 yen ($2.50) a month for extra perks such as more photo storage, blog design tools, and permanent message archiving.

This year, however, for the first time, users accessing Mixi through their mobile outnumbered those going through their PCs. The number is set to rise, especially as mobile users tend to be much younger than Mixi’s PC users. Fifty-eight percent of mobile users are under 25, as opposed to 43 percent of Mixi’s PC users.

The site has made some steps toward the mobile web. It added video this year, and partnered up with Softbank Mobile. Still, other mobile sites are gaining ground. In particular, mobile gaming site Mobage-town, which doesn’t even have a web site—has signed up 8 million members. The service lets them send avatars through games and link up with friends. It also has its own currency and users can buy goods for their avatars. Owner DeNa claims it has 50 percent penetration among the country’s teens, and some 390 million page views per day in August, compared to Mixi’s 175 million.

• PCs Pushed Aside In Japan By Mobiles, Game Consoles Etc: Report