London, Feb 16 The Angry Birds game that has been downloaded 500 million times has been launched as a free version on social network Facebook.
In the game, players use catapults to fire infuriated birds at pigs hiding inside buildings made of planks.
The Facebook game, however, allows users to pay for extra birds and other in-game extras.
The website game is similar to the iPhone app. The only difference is players have to use a mouse to sling birds across the game's landscapes rather than a finger, the Daily Mail reported.
Peter Vesterbacka, chief marketing officer of game-making company Rovio, said he hoped up to 40 percent of Facebook users would pay for in-game extras.
Rovio has never spent any money on advertising or marketing Angry Birds, the report said.
It had made 50 failed games before Angry Birds. The company also sells a million Angry Birds toys and a same number of T-shirts every month.