Thursday, January 26, 2012

YouTube's Small World

On Monday January 23rd, YouTube posted a very interesting statistic on their blog. According to the blog post, an hour's worth of video content is put on YouTube every second. They even provided an interactive animation to share the statistic in terms of satellite orbits, bamboo growth, nyans in a nyan cat, etc. YouTube is indeed growing fast.

Although we maybe used to regarding social networks to services such as Facebook, the video sharing site YouTube also has a social network in its videos. Using the videos as the vertices and the related videos links as the directed edges, a group of researchers [1] constructed a network graph for each data set from their crawler and made some interesting findings. They were able to find the small world phenomenon in the YouTube social network.

The small world phenomenon is characterized by graphs with both high clustering and small diameter. Instead of using the average diameter or the 90th percentile, the researchers used the diameter of the largest strongly connected component of each graph and found that it was about 8. This means that instead of the six degrees of separation, there is about eight degrees from one video to another. Given that the clustering coefficient of random graphs is about 0, the clustering coefficient of the YouTube graph measured to be about 0.29 is considerably high. Because of the large clustering coefficient and the small diameter, this gives you a sense of YouTube's small world.

It will be interesting to see how these numbers change in time, especially as the number of vertices in the graph grows. How many hours of video content have been added to YouTube since you started reading?

[1] Xu Cheng; Dale, C.; Jiangchuan Liu; , "Statistics and Social Network of YouTube Videos," Quality of Service, 2008. IWQoS 2008. 16th International Workshop on , vol., no., pp.229-238, 2-4 June 2008
doi: 10.1109/IWQOS.2008.32

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