Categorizing Content and Defining the Word "Similar"

Flickr tries to define similarity for photos; is this applicable to social media?

What does it mean for two pieces of social media content to be similar? Are people who liked an angry tweet interested in reading another angry tweet? If it's about birds do they want to see more birds, or is it because it's a tweet with an image, and they like nice images? What does it mean when you say you want to see more content "similar" to what you just experienced? Listen to my first wave on Anchor (posted to Soundcloud) where I discuss a recent post on the Flickr Code blog.