Every person with their Xbox Live gamertag listed has been sent a friend request from the account "Smashboards". This will serve a a dummy account you can use to have everybody's gamertag without actually being friends with them. Using the "Friends of Friends" feature added to Xbox Live in fall 2007, you have the option to look at your friend's friend list when viewing their gamercard.
If you don't understand, it'll work like this: instead of adding everybody's gamertag, you'll add Smashboards. On your friends list, scroll down to Smashboards (it'll always be offline, unless you catch me on it having some fun getting achievements or boosting in Halo), press A on Smashboards, this will open up the gamercard. Tab right and one of the options on the screen will be "View Friends". You'll see everybody who's accepted the invitations I sent out listed.