How do they know if they got every instance of person-to-person spread? E-mail, IM (3 different ones), Google+, personal communication - that's how I get the ju

How do they know if they got every instance of person-to-person spread? E-mail, IM (3 different ones), Google+, personal communication - that's how I get the jumping cats of the internet. Unless you monitor all of that, you'll get an anti-spread bias. The only way to be sure of the actual spread graph is to monitor an invite-only service, which of course might not be 100% representative of other kinds of services/products.