Even so, you can't argue that the growth is exponential by simply looking at the graph. It could just as well be polynomial. To conclude that it is exponential you need some kind of analysis, like "every user brings two, those bring two each etc".
That growth eventually slows is a very very simple point, but it's important to never forget. Maybe everybody is making this too complicated because Saul Griffith is supposed to be a 'genius' now?