> To ensure better performance, my Internet provider keeps a local copy of the popular YouTube content (caching), and when I watch a trending video, they send me the stream from their local cache. However, if I request a video that’s not contained in their current cache, I’m sent over the broader Internet to the actual YouTube content servers.

I have no doubt that the vast majority of American ISPs have cached Gangnam Style. In the US at least, Google/YouTube likely assists in this process.