HubSpot held the world’s largest online marketing seminar with Dan Zarrella, HubSpot’s Scientist of Social Media, presenting on The Science of Social Media based on his new book “Zarrella's Hierarchy of Contagiousness: The Science, Design, and Engineering of Contagious Ideas.” The webinar was held on Tuesday, August 23, 2011 from HubSpot’s headquarters in Cambridge, MA, but there were people watching around the world.