Realcases,
It is amazing what people find interesting about some videos. When you think about it though its not surprising. It's a known fact that people spend most of their time online either watching video or communicating through various channels. I think most of the time people watch videos to be entertained. Also, most of these Harlem Shake videos are short (30 seconds or less) making the commitment to watch them not that great. Mildly entertaining + National Media attention + Short video = lots of views.
Best,
Shawn