I actually find facebook to be the most helpful, and then maybe twitter. I must admit I don't really use instagram as much as I should. And Tumblr... well, I don't use it enough in general, but it's something I would like to utilise. I would love to say that I use reddit successfully... but that one requires careful treading.
But yeah, facebook is my jam. I have a fanpage, which is not huge, but I can share the video there, and then share that from my personal facebook, and I have friends who will share it out again. I find it's easier that way.