I was early investor in prosper.com one of the pioneers in peer to peer lending.. I managed to lose a modest amount of money ~10% over 3 years (which considering the stock market performance over that period of time isn't as bad as it sounds.). I think that the lendingclub.com business model makes more sense than prosper.com
If you are trying to make money, I think best case you would say that is an unproven model. Worst case it is a creative new way of losing money. In fact just last week I received a notice that I am part of class lawsuit, which is suing prosper.com and its officers for way more than they have, which alleges that Prosper.com executive did all kinds of awful things including selling unregistered securities to naive buyers.
In the case of microplace.com and kiva.com the emphasis is more on doing good for others rather doing well (financially) for yourself.
In all case there is a certain amount of hubris involved by the participants. Essentially that I am able to make more informed decision by reading a paragraph or two about the borrower, and relying on a few statistics than a bank (in the case of prosper.com or lendingclub.com) or an NGO with operations in country (for kiva.com or microplace.com) could do with systems and trained folks. I can argue both sides, but I have become skeptical.
Before putting serious money to risk it is worth reading threads here about prosper.com and kiva.com, also on Motley Fool, and doing some Googling on peer to peer lending.