For the recent two years, YouTube has already become one of the most successful VOD systems. Since it is established in early 2005, the number of the visits to YouTube not only maintain a high level, but also steadily increases even these days. It is widely believed that the online social networks contribute to this success. To study one new aspect of how much social networks contribute to the popularity of videos in YouTube, this paper provides an in-depth study into the effects of the external links in YouTube. Our study shows interesting characteristics of external links in YouTube. The existed studies reveal that `Music' and `Comedy' videos are the most popular ones. Compared with this, our study shows that the `Sports' and `Science & Technology' videos gains the most largest proportion of views from external links. Also, the percentage of views from external links reduces with power law. More specifically, we find that the top-5 external links contribute the popularity of the video most in the first four days and percentage can be up to 25%. For the video over than one years old, the top-5 external links are also able to contribute a stable part at about 4.5%. To end this, we hope our work can serve as a initial step for the study of the external environment.Department of ComputingRefereed conference pape
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