A Nielsen/NetRatings survey found there were 17.2 million unique Facebook users in Japan in May, more than double that a year ago, NewScientist.com reported.
Tech-savvy young people say they're leaving Japanese social networks such as Mixi, on which most users are anonymous, in favor of Facebook and other networks where personal information is shared freely.
Kenji Shinozaki, a 19-year-old hair stylist from Shizuoka, said he has has swapped Mixi for Facebook.
"Exchanging personal information such as phone numbers was a no-go and anyone who did risked having their account erased," he said. "We wanted a bit more freedom and openness and Facebook offered that."
Facebook Japan estimates it will pass Mixi, which has 20 million active users a month, by the end of the year.
"We think that Japanese Internet users have started to realize the fun and merit of real-name communication on the Web," said Wakaba Takemura, spokesman for Facebook Japan.
The simplicity of Facebook is winning some converts.
"The 'like' button is simple, but very clever," said Ayumi Matsuda, a first-year university student who joined Facebook three months ago. "It doesn't require a lot of thought but has a certain emotional meaning attached to it.