I think you have to build a community that is happy with your gaming website. If you already have your community will no longer be a headache to promote, how to build a community?
1. Create a fanpage on facebook and then try to advertise, or target you should get a "like" a lot
2. Create some event whose purpose so that people can invite friends to play immediately, in exchange for a bonus in-game instance.
3. Use twitter, followers and updates continue to multiply news about your gaming website.
Maybe you'll spend a lot of cost in the beginning to build community, but if you succeed you will get a reward at the end. see the biggest game in facebook for example farmville, how farmville built a loyal community without having to promote the game again and again.
Hopefully useful, and good luck.