AFAIK, Prosper didn't go out and borrow tons of money (because nobody would lend it to CL). Prosper probably just has your run of the mill corporate small business debt but the loans are not part of that. That's "committed cash" which just flows through Prosper but legally that money has to be distributed to lenders.
The quarterly report says there's $1.7M of liabilities (including accounts payable, long term debt, etc), and the most recent investment from Nigel Morris took the form of a "bridge loan". This loan is intended to be converted to stock when a bigger investor comes along. However if no bigger investor comes along, then Morris' investment is a loan, and they would enter bankruptcy with $1.7M+$1M of liabilities in addition to the "notes" sold to the lender members.
Of course that $1.7M number was as of Sept 30, 2009. Might be higher now.
So yea, you're right, they didn't borrow "tons" of money, but they do have nontrivial liabilities.