So, only 225k players finished 3.3 story, eh? Which can be done actually in 1 day and takes no effort at all.
Every single one of these surveys has shown a surprisingly low number of people who've actually finished the most recently storyline missions. That's not a good indicator at all of active players (which, like you, I honestly find surprising).

I've followed each of these though, and this certainly isn't a new thing.

EDIT: This survey puts the active playerbase right around 500k, which sounds about right given how late we are in 3.x's cycle. Seems down slightly from where we were before HW, but not by much.