It's suspicious that it's a graph of cumulative downloads. Cumulative graphs always go up nicely. But what matters is downloads per month. Someone with good monthly numbers will rarely resort to showing cumulative ones.
It's particularly suspicious the way the number for December, which we're only 6 days into, lines up exactly where it would if they were seeing hyperlinear growth. You'd have to time your post almost to the day to make it look so neat.
I would be very interested to see an ordinary graph (i.e. without graphic tricks to make it steep) of downloads per month. That would show us what's really happening.
I wouldn't be surprised if what that graph showed was linear growth (which is bad, because it means a decreasing growth rate), followed by a massive spike in December due to a special promotion.