It's a graph showing HFCS members and the users who follow us. Each circle represents an individual user, and each line between users shows the association between them (in this case that one user is following the other). For example, in the link to the interactive version you can see a huge number of yellow circles surrounding DeeJ with lines only connecting them to him. That means those users are only following DeeJ and no other HFCS members. If you look just up and to the left of where DeeJ and his followers are, you'll see a cluster of purple circles - users - who follow both DeeJ and urk, since they have lines connecting to both of them.