how do youtube get the "first referral from" facebook, twitter,etc?

Does anyone know how the youtube is able to determine if a video is coming from youtube or twitter? For example, looking at the video stats for http://www.youtube.com/watch?v=UrbHykKUfTM , youtube is able to give the number of counts fo开发者_JS百科r referral from facebook. Here is a link that gives referral from twitter: http://www.youtube.com/watch?v=26d9HLsURPM

I look at the URL from youtube's twitter share button, it don't seem to have any URL parameters that can alert youtube of where it is coming from.

http://twitter.com/share?count=horizontal&original_referer=http://www.youtube.com/watch%3Fv%3DUrbHykKUfTM&text=Check%20this%20video%20out%20--%20X-Men:%20First%20Class%20-%20Official%20Trailer&url=http://www.youtube.com/watch%3Fv%3DUrbHykKUfTM&via=youtube

---

HTTP has a Referer: field; considering the URL has the same misspelling as the HTTP spec's field, that's probably where they're getting it from.