Why did Paperbackswap grow suddenly?

Commentary on the BookMooch blog about Paperbackswap.

Excerpt: I’ve been looking at Alexa.com and compete.com to see how BookMooch is doing compared to its main competitor paperbackswap.com. For about the first two years, both sites were growing at about the same pace.

Then, sometime around January 2009, paperbackswap started doing quite a bit better, and they seem to have maintained their growth rate.

I’m wondering: why?

What did paperbackswap do that suddenly made them grow?