number of DD's is falling?

I wonder if they've just been more proactive about culling inactive people who they can't contact over the last year. It's possible that the drop isn't really all over the last year, but is accumulated inactive people over the last few years that they've just gotten around to removing from the list.

What seems more concerning to me than the drop this year, is that the total seems to have been essentially stagnant since 2002, recent drop included or not.