Bruno Goncalves, Nicola Perra, Alessandro Vespignani, Validation of Dunbar's number in Twitter conversations. arXiv:1105.5170v1, 25 May 2011
"...videos aren't books because the book is an inner experience, we construct it in our imagination, while the video is just something that we observe".And that's the problem. In essence, video externalizes "experience", reading internalizes it. Video simply isn't engaging enough. Even YouTube is becoming a sit-back medium which gets in the way of deep engagement.