it appears the Raspberry Pi population is approaching 1 million units,
Well, I tend to be skeptical of vendor claims.
But, to make an apple-to-apple comparison and given the Arduino model, you will have to compare Arduino-like devices vs. Pi after the same time since their introduction. I don't know how many Aruidno/Arduino-like devices have been sold but my sense is that its eco-system is far bigger than that of the Pi's.
But that comparison still misses the point: Roku2 vs. Pi.