Which brings us full circle to the OPs question... Anyone know any other outfits besides Alexa who tracks sites?
I did check out Prosper.com at
www.compete.com (thanks, personalloanportfolio)
Interestingly, Compete.com offers a completely different story than Alexa:

Here, we do not have a portrait of a company (or company site, I should say) in trouble. Just stagnation.
Connect.com doesnt have a 3 year graph for easy comparison, one year Nov. 06 to Nov. 07 to compare.
Instead of starting at a peak and trending down like a dying stock, like Alexas graph, this one starts low, and trends up (although it shows fairly flat growth since July 07.
Prosper Days is the hump towards the left, and does appear to be the traffic high point.
Im not sure which stats are more accurate, or tell a truer story, but one trends down, and one is flat and hasnt quite managed to surpass its peak. Neither down or flat are good signs for a growing companys website. I bet eBay wasnt flat or trending down back in the early days... (dont know for sure, though)