I have a large amount of Goodreads friends, often IRL friends with whom I know I share hobbies and interests. And yet, when I use the Goodreads "See books in common" feature, we share almost no books. I think a real problem for media-rating websites today is this: the glut of readily accessible content on the internet makes it less and less likely that any two people will share any one item of content.

The same problem can be found on Last.fm, where there are so many bands out there today, so much free listening to do, that even ardent lovers of a particular genre might not have very overlapping tastes. Last.fm appears to have been forced to change their compatibility algorithm, so that now you are "Super compatibility" with another user by just sharing a handful of bands in common, whereas in the past you would have had to share a lot more bands and listens in order to be ranked as Super.