YouTube (and most other apps) not possible until Apple release the Siri sdk to all developers early next year.
Apps like Netflix fall under universal search, so if you ask Siri to search for a film, it will return matching results for all currently compatible apps including if it's on Netflix. Just click on the film and you'll see a row of buttons showing where you can watch it. If Netflix is one of them, just click on that button to be taken to it.