- There is still need for more data. Prosper opened its "doors" in February 2006, which is less than 2 years ago. (Loans before February 2006 are between coworkers, families, friends, but not between strangers over the Internet, so the demographics changed radically on February 2006.) As much as I like horror movies, eh, Pninen's charts of 1+mo late & worse, we still don't know how the loans will perform during the third year.
Absolutely, but I'd imagine that chance of default in year 3 would be at LEAST equal or lesser than in the first 24 months, don't you?
- Even if we did have three years of history, we don't yet have a history of performance through economic cycles. Generally, during economic recessions, lower-quality loans default more. Since we know Prosper loans default much more than Experian's "average annualized default rate" for all card products, add the stress of a sour economy (sub-prime melt-down, anyone?), and even the best estimates are out the window with this systemic risk.
The same could be said about any asset class that has a limited performance history and/or exists in the present economy. I'm not going to argue re: recession, though I disagree.
- Knowledgeable bidders may end up cherry picking some of the loans that are "outliers" within some of the slices, which could cause some distortion in the numbers. This is probably more likely in the new Portfolios and result in higher default in the portfolios as planned as some of the better loans get bid below the portfolio's bidding rates. In any case, in a fluid market, as perturbations are introduced, distortions occur, so it is often a guess on how significant the distortion will be and sometimes there are "unintended consequences."
Knowledgeable bidders = who?

I know what you mean, but... outliers being cherry picked just means that "cherrypickers" will get "better" loans at lower rates, and the Portfolio people will get "less better" loans at slightly higher rates. In aggregate, I don't think it's a big deal. Might be a wash performance wise - and/or might lead to more originations (which would go to your point below re: growth).
- In addition to the distortions mentioned above, Prosper has been tweaking its site, so we have even less history on the newer additions to the "additional credit data" and other changes.
Yes, but, if anything, the addition of more credit data can only have a long-term positive influence. There may be short-term flux, but I'm happy to trade that for long-term gain..
- It is said that you have to pick your bait to catch the type of fish you want. The effects of advertising, if any, may affect the demographics of future borrowers, which may affect where within the various slices the new borrowers will fit in. (Remember: a slice tries to group similar groups of listings together, but they don't have identical numbers, so there are variations within each slice.)
Certainly - but, given the types of (free/internet) advertising employed so far, I don't see that any active/traditional marketing could make things any worse, borrower-pool wise.
- There is also the risk that Prosper may go under. (Witness the 2-month slide to a 6-month plateau as seen on RateLadder on the Prosper Loan Growth by Month chart, and that plateau is about one-fourth of what Chris Larson said would be needed to make Prosper profitable). If Prosper Marketplace, Inc. goes through bankruptcy, contracts could be rewritten by the courts that might include increasing servicing fees paid by lenders (and thus decrease lenders' profits). I don't know if it will happen, but it is a distinct possibility.
Going-concern issues are always "possible". How "distinct" such a possibility might be is subjective. Growth rates are absolutely something to keep an eye on - though I'm not too concerned, this early. Am somewhat confident in Prosper's ability to secure VC - don't think anyone went into this thinking it was a 2 year experiment to enter a multi-multi-billion market - esp. given the ability of all of its recent competitors to do same - with more and more on the way every day...
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At least they are using Prosper data, which is more realistic than Experian's "aveage annualized default rates" they were showing before.
Indeed.
-t