Where is your evidence that the rate of fans should grow exponentially?!! I WANT FACTS!!! How do YOU know its not a linear relationship? Hmm?
The only way I can think of it as being exponential is if you had 2 fans that each told another, making it 4 fans, and then 8, etc. But still, that's assuming the primary way for bands to gain fans is by word of mouth. I don't see how excessive touring correlates to an exponential increase in fan-base down the road.
I really don't care if the statistic is a good one or not. I didn't even come up with it. You should be comprendoing (I guess the verb should be explicar, but w/e) the original guy who brought it up (even though he did say the statistic itself wasn't perfect and was flawed, so he's not at fault either). My point was merely that someone used a statistic to prove a point and I felt they left out part of the story.
Oh, and your statement based on the last.fm data: "So in other words, Agalloch has 3.44 times the number of Blackguard fans NOW" is totally wrong.
Lets review some math:
Agalloch current fans (aka listeners): 171457
Blackguard current fans (aka listeners): 9053
(Agalloch fans)/(Blackguard fans) = 19
Thus using basic mathematics, the correct statement would be "So in other words, Agalloch has 19 times the number of Blackguard fans NOW".
Comprende?