Like others have commented, I too am very certain Youtube uses IP address. My family and I are behind the same cable modem, and oftentimes my family's Youtube "related videos" will start popping up in my Youtube home page, and vice versa, even though I have no interest in or have watched those videos.

Interestingly, I seem to recollect this happening only in the past few months or so, maybe a feature they turned on recently?