The Steam client was first made available for public beta testing in January 2003 during the beta period for Counter-Strike 1.6, for which it was mandatory to install and use. At the time, Steam's primary function was streamlining the patch process common in online computer games. Steam was an optional component for all other games. 80,000–300,000 gamers tested the system when it was in its beta period. The system and website choked under the strain of thousands of users simultaneously attempting to play the latest version of Counter-Strike. In 2004, the World Opponent Network was shut down and replaced by Steam. The online features of games which required World Opponent Network ceased to work unless they were converted to Steam. Source