Perspective is amazing right? Like, sure those numbers dwindled in the (let's call it the collapse because lore reference) after people dropped this game when they realized it was basically a P2P early access beta. The golden age of course when this game flourished and had that high number of registered users. But it's still almost at a million active players who played a game type that's only around during the weekend. Sure that's not really accurate for how many play on a daily basis (since some might have just gotten on destiny to play trials then quit again for a week) but it's still something that shouldn't be laughed at.
To put it in perspective: WoW like op said has almost 5million active users. Destiny almost had a fifth of that WHO JUST PLAYED A PVP EVENT. That's not counting those who played during the weekend and skipped trials for any number of reasons.