为什么会出现这样的情况，又是另一个有趣的课题。
The interesting question is why this phenomenon should occur.
那么对于像youtube和flickr这样的社会性网络服务又是什么情况呢？
如果说上述情况不错的话，那么这样大规模地、计划缜密地努力要在某一特定时刻冻结历史的动机又是什么呢？
Supposing that the mechanics of the process have been rightly described, what is the motive for this huge, accurately planned effort to freeze history at a particular moment of time?
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