I made a dumb-ass recommendation system using a graph database. The graph had links between all the "likes", "relationships" and anything else I could glean about the demographics of the user. Nodes were the items of interest and the users. The recommendations where just a walk around the user object, and ranking by how many times it hit on an "interest" node.

I thought at the time: If I could ML better, here is where it'd go. ML over the graph to draw inferences. But, alas, I was much too stupid to try it.