Those aren't really comparable links. The Backbone link goes to a "who is using Backbone" page. Which is nice, and quite convincing that Backbone is a serious

Those aren't really comparable links. The Backbone link goes to a "who is using Backbone" page. Which is nice, and quite convincing that Backbone is a serious library used in some huge projects. The Knockout link, however, goes to a "how do you use Knockout" page. Which is also nice, and a very useful documentation source that Backbone is sadly lacking. But as a new developer, if anything, the Knockout link is a better reason to pick it. Good documentation goes a LOT further than knowing that Groupon has selected the library, in my book.