There's many ways to advertise on Xbox LIVE for a site dedicated to the platform. I have built my xbox360clans.net site just by promoting on Xbox LIVE and Xbox.com.
Start on the most popular game on Xbox LIVE right now, Modern Warfare 3. You can join Ground War or Infected lobbies, they both have up to 12 players per game. After the games, send out a message to all those recent players. You can go to recent players, click on the first one at the top, send a message. Once in the message, click edit the people you want to send it to and scroll through your friends list till you find the box with a check mark in it already. Everyone below that mark is a recent player, check each box.
Make note of the last person messaged so you don't message them numerous times. People can be annoyed easily. Heck, you might even get hate mail from just sending out 1 message.
Another way is to promote word of mouth. Depending on what your topic is. For example, my clan recruitment forum. I see a clan in the lobby and I tell them straight up, Hey, are you guys recruiting? If so, then check out my site,
www.xbox360clans.net. It can help you grow your clan and learn new strategies on how to run a successful clan.
Now my forum grew to over 1600 users in just the first year. However, I also use social media sites like Facebook and Twitter that have brought in a few new users, but most came straight from xbox.