They're going bankrupt, it's just a matter of when. Prosper fails for similar reasons that bundled mortgages fail:
1) With so many interested parties, re-negotiating the loan is virtually impossible. Even worse, the potential negotiator has already been paid.
2) The underlying securities are toxic.
3) There is less information than a traditional lender would have.
4) Despite being billed as social, it's actually more impersonal. If you default on BoA, that's a large company who's not going to give you unsecured credit again and has the resources to sue you. If you default on Jane, Bob, and Joe for 50 bucks apiece, well they'll live and they can't do anything about it. It's not like borrowers were crying because lenderdude9732 was defaulted on for 23 bucks.
The early adopters came, got burned, and left. There's no more word of mouth, just the occasional newbie and a few hardcore idiot gamblers. Heck, it was sorta gambling in the beginning because we only relatively recently got 3 years worth of data on some loans! Before then any modeling and lending was based on pure speculation and wishful thinking.