I'll ask him... he's a brilliant guy by the way... and devastatingly handsome...
OK.
The ESPN Sports Poll is a scientific survey that gathers info from more than 25,000 fans each year. It's weighted and modeled to reflect the interests and demographics of the entire U.S. population.
In this case, respondents are asked if they're a college hoops fans, what "level" fan are they (avid, moderate, etc), and what is their favorite team. That top 10 only make up about 30% of responses, there's a large, long tail, not surprisingly.
Duke simply has an enormous fanbase spread nation wide. It's why they consistently draw among the biggest TV audiences every year. This has nothing to do with attendance, alumni base, etc explicitly -- it's about expressed affinity for a program. It's basically a very precise measure of a program's "brand".
Hope that helps.