Celebrity cameos along with those videos -- as well as other more recent ones like the "app for that" parody and Cookie Monster's SNL audition -- have helped Sesame Street draw over 93 million views since September. And that doesn't even include the Katy Perry segment, which, though it never aired on television, was posted on Perry's own channel and accumulated over 8.9 million views.
The most popular video is "the Elmo song," which was first posted in February of 2009. That video was featured on the site when Elmo sat down for a YouTube interview earlier this fall, which helps account for the sudden uptick as well.
The chart below tracks the monthly views Sesame Street videos have had since January of 2009:
To get a sense for what's performed well, you can view the most popular Sesame Street YouTube videos of all-time below: