Instructions
Turn on your Xbox 360 and access the console's dashboard.
Log in to the gamertag that you want to associate with Facebook.
Go to the "My Community" section of the dashboard.
Select "Facebook" and choose "Confirm Download." The Facebook program downloads to your Xbox 360.
Choose "Play Now" after the download finishes.
Select "Log in to my Facebook account" and enter your email address and password. Select "Login."
Place a check next to "Display my LIVE info." This options posts your gamertag to your profile. Choose "Continue" to finish.