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Mark Zuckerberg's surprise visit with Japanese prime minister Yoshihiko Noda came as his Facebook enjoys a belated boom in the world's third-largest economy.

Until recently, the social-networking site had trouble gaining traction in Japan. In the latest indication of how far behind Facebook had been -- and how far it has come recently -- Japan's most prominent tech entrepreneur-tweeter, Masayoshi Son, announced that he had just started using the service this week.

"I'm still a Facebook beginner, so please teach me all about it," the Softbank CEO tweeted Thursday to his 1.6 million followers.

In September 2010, Facebook had about two million site visitors in Japan, according to research firm Nielsen/NetRatings. Some of its frequent users were Japanese people who previously studied or worked overseas and started Facebook to keep in touch with their friends in the US and elsewhere in the world.

Then, in a little more than a year, the number of site visitors surged more than sixfold to top 12 million.

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The movie "The Social Network," which came to Japan in early 2011, gave Facebook a big boost. Local TV shows and newspapers talked about the film and Facebook's popularity in the US -- though when Noda raised the movie with Zuckerberg on Thursday, the Facebook founder laughed and replied that it was "very different" from reality.

Then the March 11 triple disaster came. After the earthquake and tsunami disrupted the telecommunications networks, social-networking services such as Twitter and Facebook helped fill the void.

"The March 11 disasters made more people interested in social-networking services in general," even people who previously were unaware of such services, Nielsen/NetRatings senior analyst Yoshiya Nakamura told The Wall Street Journal's Japan Real Time blog.

Facebook's growth in Japan follows a similarly rapid expansion Twitter experienced about a year earlier, and both services now have found their unique positions among Japanese internet users, Nakamura said.

For more on Zuckerberg's trip to Asia, see The Wall Street Journal.