Recommendation systems are a deep learning perennial since the Netflix contest.
My impression of Youtube, Amazon, Tumblr and other is that the recommendation process in practice is close to useless. And this isn't because I don't want recommendations.
Moreover, in all these situations, I feel like it just be improved by asking the user instead. There are thing I'd love recommendations on and I'd be quite willing to tell these portals about my preferences. But it doesn't seem like they want to know, don't have internal search worth much, don't take my search terms account when recommending, etc.
It seems like in practice, most sites actually want the effects of not being able to immediate drill past items that leverage their crappy interface to get exposure since I assume there are extra profits in one form or another to be made with these.
The first sentence of the abstract is poor English, and the fifth contains a factual inaccuracy about a very well-known recommendation algorithm (collaborative filtering is not content-based).