Investor enthusiasm for social-networking web sites reached Japan on Thursday, turning Kenji Kasahara, the 30-year-old founder of mixi.jp into a paper billionaire as demand for the freshly listed shares raced ahead of supply.

Buyers trying to get their hands on the 7,100 shares sold for Y1.55m a piece in the offering were bidding up to Y3.15m when the market closed. No shares actually changed hands on Thursday because of the imbalance between buy and sell orders.