Online monoculture and the end of the niche. In summary: online recommendation systems tend to offer a more diverse selection, but tends to reward fewer products more greatly than others:

In Internet World the customers see further, but they are all looking out from the same tall hilltop. In Offline World individual customers are standing on different, lower, hilltops. They may not see as far individually, but more of the ground is visible to someone. In Internet World, a lot of the ground cannot be seen by anyone because they are all standing on the same big hilltop.

I wish I followed the math better. Interesting stuff in the comments, too.