There are two things to mull over here. The first is why apps haven’t taken off. The other is why more people who buy a TV capable of showing online video aren’t watching online video. Although related, they have different explanations.
If you’ve ever used an Internet-connected TV, it’s pretty obvious why apps for Twitter and Facebook and reading books or shopping haven’t taken off: It’s a lousy user experience. Sitting 10 or 15 feet from your screen and trying to interact with it is a tricky thing to do. Even if your TV has a keyboard (doubtful) and you’ve got perfect vision (most people do not) and you’re a great typist (ha!) working with text from that distance isn’t easy. The mere act of firing up those apps can be a chore compared to the ease of doing so on a mobile device. It’s a far better, more intuitive experience to use the second screen — like your tablet or phone — while you’re in front of the TV. Which is exactly what people are doing.