Could just be a graphing anomaly, or that there aren't many people living on either side of the border in those areas.

I've been in that area. I think it is real.

It is a lot easier to move around and work on one side of the border or the other, which leads to people naturally not meeting people on the other side.

In the east there are so many more people and there is so much cross border traffic due to trade (particularly from the auto industry) that a lot more cross border connections exist.

I've been to the Eastern part of the border: Stanstead, Quebec/Vermont. Over there, some of the buildings are right on the border. One of the guys I worked with lived in apartments which had their parking lot technically in the US.

http://en.wikipedia.org/wiki/Stanstead,_Quebec#Geography

Check out the library:

http://en.wikipedia.org/wiki/Haskell_Free_Library_and_Opera_...

http://www.time.com/time/printout/0,8816,948520,00.html

As for the west coast, let's not forget about the geographical anomaly known as "Point Roberts":

http://en.wikipedia.org/wiki/Point_Roberts#Geography

On the other hand, if you were to think of the kind of people in each country who wouldn't have any friends at all in the other country, wouldn't they probably live around there?

The other transnational gap I can notice is the India-Pakistan border.

That big gap between the two is actually the Thar Desert.

Indeed but remember that gaps only appear on the if there's a lack strong lack of social connected between the regions. So it's entirely geographic.