Determining the most influential users of Twitter is probably not what the creators of the Cray XMT supercomputer had in mind when they designed their machine. But when you're packing this much computational heat, you go where the hard problems are. Twitter, Facebook and the rest of the social Web have become the modern-day equivalent of the water cooler, albeit with an automatic transcriptionist present. And processing all the data that conversation generates turns out to be a very hard problem.
For example, as of February 2010, Facebook included 400 million active users with an average of 120 "friend" connections each, all of whom collectively shared five-billion pieces of information in a single month.
Figuring out who the "influencers" are in such a massive social networks requires creating a gigantic social graph, where each user is a vertex and the connections between them are lines. Ranking users within such a graph requires a determination of their "centrality". That is, how many other people are connected to them, and how many people are connected to them, and so on, until you get to the trunk of the tree structure underlying connectedness on a service like Twitter.
From Technology Review
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