A YouTube Best Friend is a common YouTube user that you share interests with ie. Movies, Music, Humor. Your YouTube Best Friend often shares videos with you, puts clever comments on your videos, or sends a friend invite or vice versa. Your YouTube Best Friend and you don't usually know eachothers first names and exclusively "know eachother" through YouTube.
Patrice: Who'd you get a personal message from on YouTube? That's weird.
Dole: Oh it's probably from famguyfan666. We send eachother funny-ass family guy clips.