Hours spent playing WoW typically reaches a peak right after an expansion releases, then gradually declines. The decline this time, however, looks like it's much more rapid.
On XFire, weekly hours spent playing WoW are down over 50% from the week of the release:
Week Ending | Hours Played
Sept 30 | 67,659
Nov 27 | 32,617
(this is not a Thanksgiving weekend effect, btw; the decline has been inexorable for the past two months.)
See Nosy Gamer's blog for this data and a spreadsheet with other weeks, as well as data for other games:
http://nosygamer.blogspot.com/search.../digital_dozen
Warcraft Realms also shows activity data. It's noisier and less reliable, but appears to be tracking the XFire decline
http://www.warcraftrealms.com/weeklyfactionactivity.php
(Compare to this archived image of the decline after the release of Cataclysm)
http://inanage.files.wordpress.com/2...vitygraph2.png