The anniversary of the 9/11 attacks dominated social media from September 12-16, registering as the No. 1 subject on blogs and Twitter and among the top YouTube videos, according to the New Media Index from the Pew Research Center’s Project for Excellence in Journalism.

And the way the story played out last week out helps illustrate the different functions of the three online platforms.

The conversation on blogs, often a forum for debate, was passionate and ideological in response to columns criticizing how Americans were treating the anniversary of the attacks.

On Twitter, a primary vehicle for passing on information, users mostly shared a dramatic photo of the two streaming spotlights at Ground Zero.

On YouTube, an eclectic platform for sharing everything from dramatic images to humorous commentary, two very different videos were prevalent last week. One was straightforward