First I get list of all the followers for each person who posts here then arrange everyone in a graph (which you can imagine being like [url=http://anet.sourceforge.net/docs/user/fig2.png]this[/url]). I then calculate various statistics from it, one of which includes [url=https://en.wikipedia.org/wiki/Betweenness_centrality]how well a given node (an individual user) is connected to all the other nodes[/url]. I then use the value generated for a given node as a rank which determines how large it appears in the visualisation (the picture). The followers a given user connects to don't have to have posted in this topic. I also don't consider the followers of followers.