Mixi

It seems that Mixi is talked about a lot among people now in Japan. My friend at school mentioned about it the other day and just on the way back home after that I picked up a conversation about mixi between two girls standing in front of me on the train. Then I got an invitation email to mixi from my friend in Kyoto. What a coincidence.
Mixi is a social networking service (but only in Japanese). You can only be a member when you got an invitation email from a mixi member. You can choose if you limit publication only to the friend or to friend's friends or everyone. You can also find a friends with same interests and make comunities with them.
I haven't made a full use of it yet and only reading my friends blog on it. So I'm not sure why it became so popular but now the users are counted over 400,000.