You have to take into account that a launching period (first few weeks) EVERYONE plays hours if not for days non stop.
That was the failure the D3 haters made by pointing to Xfire stats: a dude playing 10 hours a day will only play this amount of time for several days at launch.
The moment the players drop down from 10 hours playing sessions to 3 hours and even playing every other day for the more casual ones, you automacially drop with 70-80% in the activity charts.
Also: Morhaime mentioned a few times that WOW had one of the highest long term retention rates in the industry ... but that even in WOW only 30% of all trials resulted in long time players.
As Diablo 3 is not even an MMORPG, every copy sold is still a potential player in the future as the game doesn't have a gear competition or attunements or reputations to worry about to play the game at any level.
2 corrections here: The correct numbers are 1800+ launch (introduced with patch 1.03) and 650 + now for EU in prime time.
Also the shown public games number = ONLY NEW public groups being made on a minute per minute basis. NOT included are the FULL public groups, the private groups, the solo players.
Here is the defintion from Blizz:
"What does the number displayed in the Public Games window actually represent?
The number that displays in the Public Games window represents the total number of games currently available to your hero, based on your game region and mode (Hardcore heroes qualify for a different set of Public Games). The number does not represent private games or Public Games that already have four players. Players hanging out in the character menus or using the auction house are not considered here either"