The mobile site is about breaking beyond the early adopters and iPhone users into the much greater mobile-Internet using public.

Social networking is already big on cell phones -- 11 billion of the 15 billion monthly page views on leading social networking site Mixi come from cell phones -- so Twitter's job is less about persuading people to hop on their phones and more about attracting users and gaining critical mass.

In a broader sense it's also a test bed for future mobile offerings in other countries.

"A very big focus for us is mobile, which is why Japan is very interesting to us," said Biz Stone, co-founder of Twitter during a launch event in Tokyo on Thursday evening.

"Japan is obviously very advanced in mobile so it's a unique opportunity to see how the mobile users in Japan are using Twitter," he said. "And it's also a test for us for the Asian market because it's usually very hard to localize in Asian languages. It's harder than French, German, Italian or Spanish, which we are going to be doing in the coming months. It's largely experimental but it's also a lot of fun."