Your analysis seems on point to me even from someone who doesn't really use the services. YouTube definitely doesn't promote conversation of any kind. It's a platform for consumption with essentially no standards applied to what's on it (contrasted with TV which has at least some bar). I can't speak on Facebook, but Twitter is also lacking in conversationality. Replies are limited in length and the way they're displayed means that you have to go out of your way to see replies to replies. So, that's why both YouTube and Twitter are closer to "selfish media"