The Instapaper blog details InstaRank - v1.0:

To fully shape InstaRank, we explored questions like “Did many web pages of this domain get likes, or did just one article get many hits?” and “Have links of this domain performed well historically, measured in terms of likes, reads, saves?”. We found users prefer some web domains or sources of information more than others, and used visualizations like the one shown below (which shows the top 18 domains that Instapaper users most interacted with on Sept. 4th, 2013) to identify what domains might generate popular links.