OK. Spreadsheet is attached. I was amazed by these numbers.
Specifically, of the 563,937 members who have ever signed up for Prosper, only 34,956 (37%) have ever placed a bid.
The spreadsheet has 5 columns:
(1) Days Since Registration: 0, 100, 200, ..., 700; the earliest registration is from 780 days ago.
(2) # Members: how many members registered that long ago
(3) # Members w/bid: how many of the members who registered that long ago have ever bid
(4) # Members w/bid
in past month: how many of the members who registered that long ago have bid in the past month
(5) Percentage left: (4) divided by (3) as a percent
Anything else people want to see, let me know.
I am reasonably confident that I did the calculations correctly, but I'll be happy to re-verify anything that looks suspicious.