If you're not from the US, this undoubtedly sounds somewhat trivial, but this is a long-lasting debate in the US over a product that, for better or worse, is an everyday part of most Americans' lives.
Growing up in California, solid "soda" country, I thought for a long time that "pop" was only used by the elderly and Minnesotans. Clearly I was wrong: pop is huge. Looking at the responses to the survey, there are also a fair number of indecisive fence-sitters out there that use the redundant term "soda pop" and others that prefer other terms like "soft drink" blurring the boundaries, but the geographic pattern is clear nonetheless.
The fact that "Coke" has become a synonym for soda as a whole (an intriguing instance of synecdoche) in the South is, to me, a surprise. If I ordered a Coke in a Memphis diner and the waitress asked me "What kind?" I'd assume she was asking "Regular or diet?" The idea that "7-Up" or "root beer" would be equally acceptable answers blows my mind.
There are a few as yet unexplained mysteries in the Pop vs. Soda data set:
Mystery #1: The St. Louis soda bubble
Soda is predominant in California, Arizona and the Northeast, with pop and Coke fighting it out in between. One notable exception is a large soda bubble in the middle of the country hovering over St. Louis and surrounding counties, wedged between Coke to the south and pop to the north. What accounts for this?
Mystery #2: Chicago Pop vs. Milwaukee Soda
Less than 100 miles separates downtown Chicago from downtown Milwaukee, and yet one favors pop while the other favors soda.
For a brief moment, I wondered if beer might be explain both of these mysteries somehow, as both St. Louis and Milwaukee are big beer producing cities. Sadly, I can't find any strong reasoning to support that connection (although the German word for soda is, if I'm not mistaken, "soda"), and Golden, Colorado, home of Coors, sits in a decidedly pop-favoring county.
If you haven't taken the survey yet yourself, I'd strongly recommend that you