I noticed this network-effect with one of my Facebook apps, also. It really took off in Toronto, ON and after that I'd estimate that 10% of my users were from Toronto and at least 50% or so were from Canada in general. Google Analytics always indicated Canada as the most popular country of origin.

No matter what I did I couldn't seem to level the metrics back to the US, and this was on an app that now has ~1m users.