Left 4 Dead 2 spiked up to 200% in player-base value on that weekend, making it a very popular game, and as you can see many who do not have this game. Want to play it.
Left 4 Dead is dying of players, we need them. Valve needs to initate a Free Weekend on Left 4 Dead. See how much the player-base increases compard to Left 4 Dead 2. I can almost guarantee that the player-base will increase also 200%. Doubling the amount of games we can get out of it. Free Players are also new, and will dedicate themselves to the game.
If all else fails, why not turn this game Free to Play? We have Left 4 Dead 2 with the exact same content. I say do what Team Fortress 2 did, that or start supporting the game again.