Videogum goes through the fast decline of the viral video, concluding that its last great year was 2006:
They were just the videos that stood out in my mind as being exceptional. You know, the viral video that is more than just some kid farting a powder cloud or a kitten that is also a ninja. The one you talk to your friends about. And what I discovered is that all of them were posted to YouTube in 2006.
I think Gawker’s reaction hit the nail and closed the case with this:
People watch a lot of TV on the computer. I mean, not a ton. Not enough to ruin broadcast television (yet). But, like, people watch Hulu and things on iTunes and various networks’ websites, and that stuff is much better produced than, like, that YouTube video where that one dude falls down. Virals were a distraction from other stuff on the computer, and now virals have just become that stuff on the computer.