The world's largest social network has thrown down the gauntlet to game developers to design an iconic social game like Mario or Halo for Facebook. Research suggests around three quarters of Facebook's 400m users play social games on the site. The challenge came on the opening day of the Game Developers Conference. "The next killer game is still out there and this game will come from you," Gareth Davis told a room of designers in San Francisco. "When we look at every major game platform, we see that there is an iconic defining game on that platform whether it's Sonic or Mario or Halo. "And while there are some great games on Facebook today, no one has yet produced the iconic game for [it]. The Facebook Mario is still out there," said Mr Davis.