I sat looking at the 2007 edition of "Map of Online Communities" from XKCD tonight, and I realized I hadn't written about this yet.

phpBB has made an appearance in the 2010 edition of that map. Some might argue four times, but I'll only claim three occurrence. See the map

In the "forums" inset at the bottom-right, you'll find just to the west of CruiseCritic, and across a bay from BodyBuilding.com and something called Forocoches, you find Bokt.nl. Bokt is most likely the largest phpBB3 forum on the Internet, and certainly the largest phpBB3 site listed on big-boards.com. It is also the longest running phpBB board in the world.

Other phpBB boards appearing on the map include JLA Forum (apparently using phpBB2!), and Rus-Chat, neither of which I personally know much about.

The fourth board that some people might claim is Gaia, but I don't believe they are using phpBB any longer. The same also appears to be true of Jogos, which is listed on big-boards.com as using phpBB.