I haven't played it, but pretty much every time I look out the window of my apartment here in NYC there are random people wandering aimlessly around my block with their phone pointed in weird directions. I have friends and family in my hometown telling me all sorts of stories about swarming amusement parks, masses of kids out in the street etc...

Genuinely viral, not a PR blitz. The network effect of human activity in close proximity, physically gathering in already crowded public spaces, and then spreading to areas of non-players is probably the key viral component on this one. Someone sees other people on the street playing it, they check it out, explore other areas, the network grows and so on.

Likely more effective virally than social networks because it's a well developed game mechanic - skill, mastery, rewards etc - and apparently it encourages strangers to interact, not just existing social circles. So the activity leaps over the "tribal" firewall.

It just turns out that viral social media manifested in the real world starts resembling an actual virus.