A new whitepaper from online measurement firm ComScore reports that Japan is the only country in the world where Twitter outperforms Facebook in terms of monthly unique visitors.

Unique monthly Japanese Twitter users number 21.6 million in Japan, representing a reach of 29.4%, while Facebook counts 14.4 million users, a reach of 19.6%. Japan’s next most popular social network is Mixi, a local offering that until mid-2011 rivalled Twitter, but has since dropped off and has just been taken over by Facebook.

But Business Insider predicts Twitter’s Japanese dominance may not continue for long, suggesting that if the current rates of growth continue, Facebook will overtake Twitter within a year or two. And when time spent on each site is considered, Facebook commands a much larger chunk of its users’ time.

One reason for the success of Twitter and Mixi over Facebook among internet users in Japan may be the Japanese preference for online anonymity. AdAge quotes a recent Forrester report suggesting that cultural anxieties about privacy have affected the uptake of social media.

JAPAN

Top 5 social networks in Japan

Contrast the above graph to Australia’s, which is a more typical scenario among the 43 countries included in ComScore’s measurements.

AUSTRALIA

Top 5 social networks in Australia

The only other countries where Facebook doesn’t reign supreme are Russia, South Korea, Brazil and Poland, although the latter pair will not remain this way for long.

Japan is anomalous in its social media profile for another reason. While in most countries social media accounts for the majority of time spent online, now surpassing even email and porn, in Japan just 58% of the internet population uses social media. The only other country where less than 85% of internet users were using social networking was China, where Facebook and Twitter are blocked, although local substitutes enjoy popularity.

Worldwide social media penetration