There is this chart making it's rounds on the internet for quite some time now. It shows player activity between 2005 and 2009 as measured by Warcraftrealms.com and its addon CensusPlus.
There's also this chart of the last year of player activity at Warcraftrealms.com.
Now, the big question, of course, is how this data correlates with subscribers. After all, everybody would like to know how deep the slump between patches actually is for Blizzard. But they insist on only telling us the subscriber peaks.
Since I love riddles and playing detective, I compared this data with the official player numbers by Blizzard between 2005 and 2009. We have very good data from this time intervall, because Blizzard was so surprised and proud about the success of WoW that they constantly told us the newest peak; often every month. The data comes from mmodata.net
Little reminder: TBC was released January 2007. WotLK was released November 2008.
The red line is the official subscriber number. The blue line is the player activity scaled with the factor 11. As you can see, during classic WoW the player activity was very good at predicting subscriber numbers. Later it tended to underestimate the number of subscribers. One reason is probably that WOWCensus only works for NA/European realms and WoW started to grow rapidly in China and other places.
If anybody has some WoWCensus data for a larger timeframe, or knows why January seems to be counted twice in that chart, please leave a comment.