This is a great question.
I think I know what that guy earlier meant when he said “YouTube” Commenting on, and interacting with the YouTube community for some is the only / best way to promote a channel. I feel the same way sometimes, because interaction through youtube it is a direct link between your channel and someone else’s channel.
To answer the question as you posed it, I am on Twitter and Facebook, and I am not completely thrilled with either. My feeling is that facebook is more interactive and has a better chance of having your content shared and viewed.
But, in my opinion Twitter seems like an odd game of “who has the most followers” I often get the feeling that people on twitter only follow others because they expect a follow back, and are rarely interested in the content you are putting on twitter. In general, because people on twitter often follow so many people I don’t think that many of your follower may actually SEE your content, it just gets lost in the sea of other tweets.
Perhaps I’m wrong, let me know what you guys think.