The amount of money that you cost them for server time is negligible - when using WWF, you're sending a small handful of web requests for every move you make, a

The amount of money that you cost them for server time is negligible - when using WWF, you're sending a small handful of web requests for every move you make, at a very slow rate, and over the course of a day an average user of Hacker News probably puts more stress on these servers than a person does on Zynga's by using WWF. I'd guess that the expected monetary value of the increased engagement of your friends due to your presence more than makes up for the cost to provide the game to you, probably by a factor of 100 or more.