webDiplomacy: 121,211
PLAYdiplomacy: 208,598
worldLeaders the Game: 622,631
www.stabbeurfou.org: 2,886,602
BOUNCED: 4,397,663
goonDiplomacy: 1,797,648
phpDiplomacy: 5,057,502
dip.xbsd.kr: 8,438,703
oli.rhoen.de/webdiplomacy: N/A
Highlight: Since my last ranking, every one of the websites, except for WorldLeaders the Game, has climbed up the ladder. Either web-based implementations are becoming more popular, users are increasingly using accounts or more than one website or more Diplomacy players have the Alexa Toolbar.
http://www.rhoen.de/ is dedicated to play web-based implementations of Diplomacy, and thus cannot be ranked, as previously claimed. It's Alexa Traffic Rank is 689,072.