For what it's worth, I plugged my own small business into there. While their traffic numbers were off by more than a bit, they had my demographic described essentially accurately. While the numbers are probably wild hindquarter guesses due to sampling issues, they correctly pegged my audience as heavily female, over-educated, caught both peaks of the income distribution (I was impressed by that one, to say the least!), and they absolutely nailed the age distribution.
Remember, though, Prosper would have audience data from two seperate, and very unequal, groups. This could give their algorithms some headaches. Example: black participation on the Internet lags white participation substantially, so if blacks are overrepresented in the Prosper borrower population they'd peg Prosper as a black-oriented (or skewing, or whatever) site pretty quickly. Similarly, we have to have a much larger rate of participation by people earning $100k+ than your average website.
(I'd be interested in seeing what TMF has. Oh wait, I can! And, holy cow, if they're right no wonder TMF is an advertising dream...)