What are the most popular Japanese social networks?

Facebook or italki isn't enough?

It's mixi. mixi is the most popular SNS among Japanese. http://mixi.jp

Mikami is right, its mixi.
But it requires you an activate mail adress of your mobile.

well...I don't think mixi is just for cell. I guess u can use mixi with any mail adress. if u were my friend, I could invite there......heehee

i havent loged it mixi for sooooooooooo long time