Again, not a huge shock; we're officially in Cataclysm's twilight (heh) years, and drop-off like this before an expansion is to be expected. It's worth noting in this case that the numbers for active raiders might be a little better than what's reflected here -- after all, plenty of guildless people have been able to raid thanks to Raid Finder, and that sort of activity wouldn't be tracked by this metric.
One thing's definitely for sure, though -- the game needs a jump start in the form of Mists of Pandaria if Blizzard wants people to stick around. Thankfully, it's right around the corner, but one wonders just how long this cycle can perpetuate. The end of Wrath and the lifetime of Cataclysm showed us that diminishing returns are already in effect, Annual Pass or no, but MoP's endgame is decidedly different than what we've seen in the past. Perhaps things will be different this time.