Obviously, there are a lot of recommender systems that will be very specific to a domain. In the past couple of years, with the massive rise of social networking tools and social apps all around us, the interest in social recommender systems has grown massively. Essentially, we are looking to make useful new connections between people that are effectively part of the same social circle, but may not have realized it yet.

Looking at the following sample network should clarify this immediately:

A social networking graph

In the preceding simple network, there is a very high likelihood that we can close some friendship loops very easily, by suggesting connections between new links between people. Very often, we will be using the graph theory principle of triadic closures, meaning that we will be closing the missing links of the triangles in the structure of our network.

So let's explore that social networking use case some more in the following chapter with a very specific...