Having played multiple MMO's over the twenty years or so [MUDs -> EQ -> EQ2 -> WOW -> RIFTS -> WOW -> SWToR -> WOW], one of the things that I have tried to keep an eye on is subscription rates. Why? Well mostly because I know when they get lower that server merges happen. So when Blizz posts their quarterly earnings I usually end up reading enough of it to take a look at their subscription rates. A few days ago they released Q1 2013, and the WOW subscription rates came in at 8.3 million.
Here is a quick historical list of their past numbers:
November 23, 2004 - World of Warcraft release date
Q1-2005 1.5 million
Q2-2005 3.5 million
Q3-2005 4 million
Q4-2005 5.5 million
Q1-2006 6 million
Q2-2006 6.5 million
Q3-2006 7 million
Q4-2006 8 million
Q1-2007 8.5 million
Q2-2007 9 million
Q3-2007 9.30 million
Q4-2007 10 million
Q1-2008 10.70 million
Q2-2008 10.90 million
Q3-2008 11 million
Q4-2008 11.50 million
note: I couldn't find Q1 through Q3 for 2009
Q4-2009 11.5 million
Q1-2010 11.5 million
Q2-2010 11.5 million
Q3-2010 12 million
Q4-2010 11.5 million
Q1-2011 11.4 million
Q2-2011 11.1 million
Q3-2011 10.3 million
Q4-2011 10.2 million
Q1-2012 10.2 million
Q2-2012 9.1 million
Q3-2012 10 million
Q4-2012 9.6 million
Q1-2013 8.3 million
Why is this worth mentioning? Well it appears to me that WOW's subscription base is in decline. It's far from dead and as long as the powers that be within the company are happy with their profit margins we can expect it to keep going. But it does mean that we're likely to see some server mergers in the near future. Especially if Q2 looks even more grim.
(source 1)
(note: 2009 through 2013 quarterly numbers compiled manually by googling for the answers)(source 3 - for 2005-2008)