@ Urbi:
Couple of things (btw: "free fall" is kinda cheap):
First, context. You know how you can chart a stock's price performance on certain sites with news events marked? You mention Prosper Days - so, clearly, you understand (I know you -really- do understand PR, but bear with me) how press coverage can have a huge effect.
Second, other datapoints: You notice how the membership growth is dropping, but the number of active borrowers and lenders is steady,
just under the all-time peaks? How would you explain that? I'd submit to you that this is not some precipitous free fall decline, but that,
instead, it's the result of direct advertisement and much more targeted marketing. If membership growth drops, but the borrower/lender actives stay roughly in line, would that suggest to you that a higher percentage of those becoming members are going on to be actives?
IE - fewer net memberships, but higher rates of participation/conversion = targeted/optimized marketing efforts at work... like, tv, radio?
(also, just for completeness, note that the referral program went dark for a couple of weeks in jun/july...)
I trust you'll let me know, good and hard, if you think any of that doesn't make sense - and I know you're qualified to recognize such.

-t