This Harlem Shake Meme was started from Queensland, Australia by 5 teenagers doing funny video, then the world starts imitating. And the term 'Meme' also means 'spreading via internet'. Hence, you can see a lot of these videos in Youtube.
I think the main reason for Harlem Shake Meme to become so Viral is that, everyone can do it. Just like Gangnam Style. And most importantly, everyone can entertain themselves by simply dancing it (Shaking).
-See for yourself-
It is popular now