All of the 5 most viewed online trailers in 2011 came from films that were in the top 10 highest grossing of 2011

"Film sequels ruled 2011, but none more than Twilight's Breaking Dawn: Part 1. With nearly 200 million views for the year, Breaking Dawn – Part 1 was easily 2011's most viral movie trailer. As with all Twilight films, fans have accounted for most of the viral activity for the campaign. With the exception of a few dozen originals, more than 1,500 of the clips are copies or derivatives uploaded by audiences.
Transformers: Dark of the Moon and Cars 2 battled for runner-up, but Transformers came out on top by 2.6+ million views. Fast Five and Harry Potter and the Deathly Hallows land at fourth and fifth places.
All of the films on this list are in the top 10 grossing films of 2011, and three are in the top 5.  It makes sense – audiences get excited about a movie and they go online to watch its trailers in anticipation of its release."

Source:  Data from Visible Measures, reported in their blog, 21st December 2011