How can you find friends on Xbox Live without knowing their Gamertag?

[Neil1976]

Is there a way to find any of my Facebook friends or Twitter followers who are on Xbox Live. I heard that Microsoft have removed the Facebook/Twitter apps so I was wondering if there is another way.
Yes I know I could ask them in real life but that could be a pain to speak to all of them one-by-one. I am not even sure which friends actually own an Xbox. It would be useful to have an app on the Xbox or Facebook that can show you your friends' gamertags.

[BOING!]

Ask them in real life

[ScσττRΛSC³]

There maybe a way using email addresses attached to the accounts. Not sure where that could be found specifically.

[Tom]

You can still connect to Facebook friends with call of duty but I don't know of any other way

[opurt]

There used to be an Facebook app on the Xbox 360 that let you do this, but Microsoft got rid of the app when they released IE on the 360. If you got the app before they removed it, then the function should still work.