Hmmmm....not much of a selection of cities. I wonder if the poll they used to come up with their data was limitted to large cities. I'd say that Philadelphia is right up there on the list, however. That's partly due to NEST. But, it seems that the area is freer than most in a variety of areas. So, it wouldn't surprise me to learn that it's number one in many of them.
My own hometown area in upstate NY probably had just as many people into tickling. In fact, I was never at a loss for people into it. I was lucky enough to enjoy tickling on some level since I was a little kid. There are at least a half dozen of us here in the community today, with many others who simply don't get involved online....and we're only a fraction of the size of Philly. Just as we have a lot of lurkers here on the forum, there are plenty out there in the real world as well.