The WoW census site WarcraftRealms has posted a player activity graph to demonstrate the wane of player interest shortly after the release of The Burning Crusade. The chart peaks at the release of the expansion and then reaches a decline, probably the first since the release of the game in 2004.

These numbers actually represent character activity, each player may have multiple characters. Only 2.3 million of the current 8 million shown are characters in the 61-70 level ranges.

Playing devil's advocate for a moment: It is possible that the decline on these graphs can be accounted for by the reduction of bank alts. The TBC expansion added more and cheaper bank space for characters, so many players may have retired their secondary banking characters since then.

My personal opinion however, does match with this graph. Like many other longtime players, my friend's list is peppered with players who have left the game recently.