Domestic social network pioneer Mixi Inc. has stumbled and the once rising star has been eclipsed by its giant U.S. competitor Facebook and domestic social game operators.

A media report Tuesday said Mixi founder Kenji Kasahara may sell his entire 55 percent stake in the company to other firms, and the rapidly expanding social game companies DeNA Co. and Gree Inc. may be candidate buyers.

Mixi the same day flatly denied that Kasahara planned to sell his stake. "What was reported is not true," the company said in a statement.